Riparian Surrogates in the Sacramento/San Joaquin Delta and Their Habitat Values[1]
Nona B. Dennis, Douglas Ellis, John R. Arnold, and Diane L. Renshaw[2]
Abstract.—The distribution and condition of riparian vegetation in the Sacramento/San Joaquin Delta has been highly modified during 130 years of land reclamation, construction and maintenance of the present levee system, and conversion of the land to agricultural cultivation. A variety of vegetation complexes with partial riparian attributes exist under a few natural, but primarily induced conditions in the Delta, providing some of the wildlife values associated with historic Delta riparian vegetation. These examples of modified riparian communities suggest that compromises will be necessary to achieve wildlife values that are compatible with flood control requirements and agricultural land use in the Delta.
Introduction
In January, 1979, a team of individuals undertook a year-long study of wildlife habitats in the Sacramento/San Joaquin Delta. The study was funded by the USDI Fish and Wildlife Service (FWS) and co-directed by the California Department of Fish and Game (DFG) and the FWS. The study culminated in the Sacramento/San Joaquin Delta Wildlife Habitat Protection and Restoration Plan (California Department of Fish and Game 1980) whose principal objectives were to:
1. document the wildlife habitat resources of the Delta, based on a year-long avian census and mammal survey as well as on available data, and to describe the human demands and activities which affect them;
2. seek more effective ways—both technical and institutional—to protect and enhance existing wildlife habitats or restore those that have disappeared from the Delta; and
3. present the policies and interests of the DFG and FWS with respect to protecting the wildlife resources of the Delta.
This paper is an informal summary of information in the Sacramento/San Joaquin Delta Wildlife Habitat Protection and Restoration Plan. It also presents the senior author's observations concerning the circumstances under which "surrogate" riparian communities have developed in the Delta and are presently managed, and the opportunities for their protection and enhancement or restoration as partial but valuable riparian wildlife habitats.
The "legal Delta" (California Water Code Sec. 12220) encompasses 2,986 km2 (1,153 mi2 ) or 298,800 ha. (738,000 ac.) (fig. 1). Prior to 1850, when reclamation began, the Delta was largely a marshland of about 161,900 ha. (400,000 ac.) surrounded by approximately 121,500 ha. (300,000 ac.) of slightly higher lands and shallow backswamps behind natural alluvial levees (Thompson 1957). With the completion of reclamation in 1930, the Delta was transformed into 60 major leveed islands totalling 182,000 ha. (450,000 ac.) of primarily agricultural land, more than 800 islets, and 1,130 km. (700 mi.) of waterways lined with 1,709 km. (1,062 mi.) of levees. A number of urban settlements are situated on the periphery of the Delta, but the predominant land use is agricultural.
The Sacramento/San Joaquin Delta Wildlife Habitat Protection and Restoration Plan surveyed all of the evident wildlife habitat-types represented in the Delta, including agricultural, urban, and other "developed" habitats. Vegetation, physiography and land use were used as
[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].
[2] Nona B. Dennis is President, Madrone Associates, Environmental Consultants, Novato, Calif. Douglas Ellis is Field Ornithologist, Madrone Associates. John R. Arnold, PhD., is Professor Emeritus of Zoology, California State University, Sonoma, Rohnert Park, Calif., and Senior Consultant, Madrone Associates. Diane L. Renshaw is Senior Consultant, Madrone Associates.
Figure 1.
The Sacramento/San Joaquin Delta.
bases for classifying habitats as they occur under present conditions, taking into account that agricultural practices (e.g., crop selection, irrigation methods) can profoundly change habitat conditions from year to year.
During the study it was apparent that several Delta habitat-types, both on and off levees, have riparian attributes (terrestrial adjacency to free waters and/or substrate with high soil moisture). However, few areas in the Delta still support historic communities of riparian species (cottonwood, white alder, western sycamore, ash, valley oak) that can be classified as riparian woodland or forest according to conventional criteria. As with many managed waterway systems, riparian vegetation exists in the Delta under sufferance and in many variations. The variations (i.e., "surrogates") are nonetheless worthy of attention, since they demonstrate considerable wildlife value and, in an area managed principally for flood control and agriculture, may offer among the few opportunities in the Delta for useful terrestrial habitat for many wildlife species.
Previous Work
Much attention in the past several decades has been focussed on the disappearance of riparian vegetation from many parts of the Delta, primarily in connection with levee construction and maintenance practices. There have been few ecological or floristic studies.
Most ecological investigations of riparian vegetation of the Sacramento River stop in the vicinity of Clarksburg, Yolo County, where the major native stands of Sacramento Valley riparian forest first appear (Thompson 1957). To our knowledge, only one analysis of structure and floristics of riparian vegetation of the Sacramento Valley extends study sites into the northern Delta, to include Stone Lake in the Sacramento River flood basin and Delta Meadows on Snodgrass Slough, as examples of relatively unmodified natural condition (Conard etal . 1977). The study characterizes riparian vegetation in terms of succession related to fluvial processes. This approach is generally valid but is not entirely useful when applied to Delta riparian vegetation. The extent of early vegetation and continuing manipulation of land, water, and vegetation in the Delta imposes an entirely new set of rules governing succession, even though familiar successional seres can still be seen throughout the Delta.
Whitlow etal . (1979) studied the role of natural and introduced riparian vegetation in retarding levee erosion. The research specifically addressed problems of establishing emergent species at the fluctuating water line. This study also provided an overview of the problems of maintaining vegetation on levees in the Central Valley (lower Sacramento River) and surveyed experimental levee planting projects conducted by the US Army Corps of Engineers (CE) and the California Department of Water Resources (DWR). The study concludes that revegetation of Delta levees is technically feasible and can be compatible with flood control objectives, if species and location are carefully considered.
Jepson (1893) catalogued species growing on natural levees along the lower Sacramento River. Mason (1957) studied aquatic and emergent species in the Delta's tidal wetlands extensively. Atwater (1979) reported on the distribution of dominant species and flora of six "pristine" Delta islets in pursuing the question of what vascular plants might have inhabited the freshwater reaches of Pleistocene estuaries. In so doing, he demonstrated the subtlety of the ecotone between "wetland" and "riparian" conditions in much of the prereclamation Delta, a condition which we feel bestows distinctive riparian attributes on the Delta.
The only complete and current mapping of wetland and terrestrial vegetation-types in the Delta was done by the CE (US Army Corps of Engineers 1979) at a scale of 1" to 1,000'. Delineation followed the FWS wetlands classification system (Cowardin et al . 1979). The system was based on a hierarchy of habitat variables: morphology (riverine, palustrine, lacustrine, etc.); vegetation-type (herbaceous, emergent, etc.); substrate (mud, sand, cobble, etc.); and salinity regime. Delta riparian vegetation under this classification falls primarily into one category: viz . "palustrine forested," or "riparian forest of broad-leaved deciduous vegetation, 6 m. or more in height" (ibid .). This riparian "type" compares with the R1 (large woody vegetation) and R1v (valley oak woodland) classifications used by the Central Valley Riparian Mapping Project (Central Valley Riparian Mapping Project 1979). A second category can be considered riparian, although CE maps do not specifically apply that term: viz . palustrine scrub/shrub, "dominated by woody vegetation less than 6 m. in height, with broad-leaved deciduous plants" (ibid .). This category compares with the R2 (low woody vegetation) used by the Central Valley Mapping Project (ibid .). The CE applies several other FWS categories to cover Delta "wetland" and "emergent" types. These generally compare with R3 (herbaceous vegetation) and M (marsh) used in the Central Valley Riparian Mapping Project.
The Sacramento/San Joaquin Delta Wildlife Habitat Protection and Restoration Plan adapted the FWS system, as used by the CE in mapping the Delta, to cover wildlife habitat designations which are more familiar and to express a wider range of riparian-like conditions.
A more detailed structural and floristic analysis of the diverse contemporary manifestations and wildlife use of riparian, or quasiriparian, vegetation in the Delta would be useful and might provide guidance with respect to vegetation, given the apparent constraints of flood
control and agricultural operations. The present paper does not fill this gap. It suggests, however, that the highly modified condition of the Delta, and its distinctive mixed estuarine and riverine origins and characters, have produced communities which have at least some if not all of the attributes of classic riparian zone vegetation, as well as many of the same plant species, and many of the same wildlife values.
Past and Present Distribution of Riparian Vegetation in the Delta
The Sacramento/San Joaquin Delta is an inland triangular network of waterways formed by the rising of sea level through the Carquinez Strait between 7,000 and 11,000 years ago to meet the alluvial fans and outflows of the Sacramento, San Joaquin, Cosumnes, and Mokelumne Rivers and smaller tributary streams (Atwater 1980) (fig. 1). Both rivers and tides have shaped the land and soils of the Delta (fig. 2). Peat soils derived from tule marshes occupy most of the level, low-lying central part of the Delta. Mineral-rich alluvial soils deposited by the rivers entering the Delta predominate on the periphery. Low natural alluvial levees defined the water courses throughout much of the Delta prior to reclamation, but today only vestiges of natural levees remain, the more massive man-made levee system having obscured these natural levees long ago.
Figure 2.
The majority of Delta islands have been cut off from floods and
tides. A few channel islands continue to be influenced by both.
Before reclamation, three-fifths of the Delta was awash with an ordinary tide, largely submerged by a spring tide and capable of being entirely overflowed by a river flood (Thompson 1957). There was little topographic relief, especially in the central and western Delta. The only features rising above the sea level swamp were the typical asymmetrical natural levees, narrow ridges of alluvium meandering into the backswamps, and occasional wind-deposited hummocks of sand, the highest of these 5 m. (17 ft.) in the western Delta near Knightsen-Oakley. Most features in the central Delta were less than 3 m. (10 ft.) high.
The prereclamation Delta "islands" were thus nothing more or less than backswamps partially or fully enclosed by alluvial levees, drained by meandering sloughs. The natural levees, both parallel and lateral to waterways, were composed of fine to sandy material carried in from surrounding Central Valley uplands by the rivers and their tributaries. The Sacramento River, with its greater volume and velocity, carried greater amounts of sediment toward the Delta than did the San Joaquin, and thus deposited more substantial natural levees.
Levee banks, shaped by tidal scour as much as by river deposition, were abrupt at the river face with the land side sloping inland, enclosing a saucer-like topography. As banks rose high enough above the mean tide level, water-tolerant
vegetation could become established, checking the velocity of sediment-laden water. This would promote the accumulation of greater amounts of alluvium, encouraging more plant growth.
Generally, the height and breadth of natural levees increased with distance north, east, and south from the low tide, low water level at the western apex of the Delta. Thompson (ibid .) reconstructs an approximate gradient of increasing levee height northwest of Sherman Island, north of Isleton, north and east of Staten and Tyler Island, south and east of Roberts and Rough-and-Ready Islands, and southwest of the latitude of Rough-and-Ready Island. These natural levees beyond the central Delta might be as high as 3 to 5 m. (10 to 18 ft.), with the highest approaching 7 m. (24 ft.) at Sacramento. Natural levees averaged as much as 200 m. (660 ft.) wide along the (old) Sacramento River and 120 m. (400 ft.) wide along parts of Steamboat Slough (ibid .).
Prereclamation Riparian Vegetation
Hydric to mesic vegetation reflected the local microtopography of natural levees and hummocks and peripheral gradients in the otherwise unrelieved flat, generally wet terrain. Several investigators have reconstructed historic conditions by studying early illustrations as well as the few remaining sites which are relatively pristine. Thompson describes the conditions that were seen by travelers going up the Sacramento River:
The monotony of the green or brown canebrake-like vegetation was broken by channel and pond surfaces and by strips of alluvial land where woody shrubs and trees and herbaceous annuals grew. This natural levee cover consisted of coarse bunch grasses, willows, blackberry, and wild rose thickets, and galleries of oak, sycamore, alder, walnut, and cottonwood.
The shrubs appeared among the tules of Sherman, Lower Roberts, and other centrally located islands, but a continuity of woody growth probably did not develop until the latitude of Brannan Island and Stockton. This cover became a belt of heavy oak timber on the upper four miles of Union Island, and probably on Robert's Island. Fine groves occupied the more southerly San Joaquin distributary banks. Similar stands of woods occupied the Sacramento River levees upstream from about the lower end of Grand Island, in places so overhanging the river that it interferred with the rigging of passing ships (ibid .).
Levees and Reclamation
Levee building and reclamation of Delta lands irreversibly altered the physical appearance and function of the area. Perhaps nowhere in California have changes to the physiography and hydrography of an area been as profound as those that accompanied reclamation within the Delta. At the same time, throughout the major and minor drainages which feed into the Delta, major land-use changes significantly altered fluvial processes of erosion and deposition, ultimately affecting Delta waterways.
The history and techniques of levee construction in the Delta have been thoroughly documented and need only be reviewed briefly here. The early "shoestring" levees were handbuilt from blocks of sod from island interiors; they were low earthen mounds, resembling natural alluvial levees, and afforded little protection from flooding. Later, clamshell dredges constructed higher, more substantial levees, using construction material from "borrow" ditches immediately external to the levee site. This practice created levees that were set back from an outer, waterside berm. When it was realized that levee survival was partly predicated on the flood-carrying capacity of adjacent channels, the interval between levee toe and borrow ditch was broadened. These residual berms also helped protect levees from rupture and wave attack (ibid .).
To some extent, the berm could accommodate a natural riverine system, the channel and narrow floodplain between levee and berm becoming part of a complementary system in which overland flow could be regarded as a natural process rather than an unnatural hazard. As levee construction techniques improved and conditions of channel alluviation and interior island subsidence dictated, artificial levees became more massive, as much as 61 m. (200 ft.) broad at the base and 9 m. (30 ft.) high, dominating the landscape. Natural levees and most of the artificial berms have either been removed in channel or levee improvements, concealed by reconstruction, or eroded away. The large levee structures which replaced them are so sloped and elevated above the natural floodplain level that with or without riprap they have lost the appearance of a natural floodplain and "riparian zone," except for obvious proximity to a waterway. Nevertheless, the levees introduced into the Delta many miles of topographic relief.
To the extent that "riparian" is an adjective that defines the topographic zone adjacent to fresh water (Warner 1979), the 1,600+ km. (1,000+ mi.) of man-made levees that now line 1,120 km. (700 mi.) of Delta channels and sloughs are riparian zones and hence existing and candidate substrates for woody riparian vegetation. The actual extent of this linear zone probably exceeds that of the prereclamation natural alluvial levees, particularly in the central Delta, which was largely a tidal marsh with little topographic variation.
"Riparian" as a concept also embraces other attributes—notably available soil moisture (high
groundwater table), even distant from waterways. As reclamation proceeded, the backswamps of the Delta were drained and levees cut off their replenishment by flooding. But the water table has continued to be at or near the surface of the land, which is 3 to 6 m. (10 to 20 ft.) below mean sea level in many areas. These conditions would enable mesic vegetation to invade most of the Delta if the land were not regularly drained, cleared, and cultivated.
Changes to Riparian Vegetation Since Reclamation
It is ironic that humans—having built into the unrelieved flat landscape of the Delta hundreds of miles of "riparian zone" within or above the normal tide and floodplain level, sufficiently elevated to support luxuriant plant growth, and having drained the swamps—have been stripping, burning, plowing, or covering much of it ever since.
Some of the early land developers planted alfalfa or Bermuda grass on the waterside of levees in an attempt to reduce wave damage. Willows were used more commonly because they survived prolonged submersion better than alfalfa. Some reclamation interests thought that the roots of willow and other shrubs would reduce the tendency of peat levees to burst when subjected to prolonged river pressure. Unfortunately, willows tended to choke out the tule, depriving levee faces of the mass of sod and stems which were thought to break the force of wave action.
The protective role of berms left in early levee construction was enhanced by natural invasion of willow, cottonwood, and tule, which formed a living defense against wave attack. The exterior borrow ditches served as alluvium traps and sites for succession from tule marsh to diverse riparian communities. Most of these berms are now gone; those remaining are restricted largely to a few reaches where setback levees have been deliberately reconstructed.
Since reclamation, the areal extent of all types of vegetation in the Delta has been steadily decreasing, becoming limited to narrow waterway margins and the outer edges of some levees, a few overflowed tracts, small unreclaimed islands and undeveloped lands outside levees, trapped sloughs and drainage ditches, some inner levee slopes, and whatever lands in island interiors have been left uncultivated or used as dredge spoil sites.
The Delta still contains remnant stands of riparian forest which, if they survived initial reclamation and subsequent clearing of land, continue to survive in spite of virtual isolation from the fluvial processes which induced and once sustained them (fig. 3). A few significant stands of riparian woodland can be found on the periphery of the Delta; there are examples near Thornton at the confluence of the Cosumnes and Mokelumne Rivers, isolated stands northwest of Stockton, and along the southern reaches of old Sacramento River and the San Joaquin River south of Stockton and Tracy. Other vestiges still exist on wooded islands, on a few remaining unleveed banks, and on occasional broad berms left on the waterside of levees.
Figure 3.
Most stands of riparian forest have been cleared from the Delta floodplain, leaving stringers
or small islands of cottonwood and associated species in a few uncultivated areas.
Much of the adventitious riparian growth in the Delta still reveals itself as a series of hydric and mesic communities only partially
related to the dynamics of the waterways, dominated by emergent marsh species, with hummocks, older levees, highly modified levees, and miscellaneous interior areas supporting shrub thickets of woody and weedy species. The persistence of certain mesic species in springing up where ever the presence of groundwater, seasonal flooding, and benign neglect permit, is evidence of the natural propensity of the Delta to support riparian plant growth.
"Surrogate" Riparian Communities in the Delta
It is unlikely that levee construction and maintenance, with flood control as its main objective, and the prevailing agricultural land use, which is totally dependent on the levee system, will ever permit further manifestation in the Delta of the rich diversity of species and complex physiognomy that characterize a fully developed riparian plant association. Significant political, economic, technical, legal, and philosophical questions revolve around the maintenance of that system. Two perennial considerations that constrain riparian vegetation in the Delta are: 1) the degree to which vegetation on levees ("riparian" because of the obvious proximity of levees to waterways) is compatible with the primary flood control function and design specifications of the levee system; and 2) the degree to which "natural" vegetation in island interiors ("riparian" because of persistent high groundwater) is compatible with intensive crop cultivation. The levee and other riparian and quasiriparian communities now found in the Delta are often natural in appearance but nonetheless are largely dependent on deliberate or inadvertent human actions.
Levee Riparian Vegetation
The four most common riparian-like communinities associated with waterways on Delta levees are shrub-brush (scrub/shrub in the Sacramento/San Joaquin Environmental Atlas (US Army Corps of Engineers 1979)), brushy riprap (also designated scrub/shrub by the CE), herbaceous banks (labelled upland by the CE (ibid .)), and unvegetated riprap, now a common condition on most maintained and reconstructed levees. Other riparian-like communities found on levees might be called urban (landscaped) riparian; monospecific riparian with such species as willow or alder; and "discontinuous" riparian, evidenced by occasional isolated trees left standing in the wake of stripping, burning, or disking.
Riparian Shrub-Bush .—Riparian shrub-brush is characterized by broad-leaved woody growth less than 6 m. tall. The most common plants are shrubs such as blackberry, wild rose, young alder at the edge of the water, willow species, and herbaceous species such as mugwort and stinging nettle. Occasional small trees—usually willow, cottonwood, or sycamore—may be present. This is a highly variable community; if left undisturbed, as on natural berms on the margin of some of the channel islands, it would develop into woodland. Periodic disturbance from levee maintenance practices generally discourages this long-term successional process, however.
One of two structural components typical of fully developed riparian vegetation may be absent. An upper stratum such as that provided by mature cottonwoods with snags is most often absent. There may or may not be a well-developed thicket and ground stratum, depending on how negligent levee inspection and maintenance have been.
Diversity of bird use of this habitat parallels its relatively lesser vegetative diversity and structural complexity. For example, hawks (e.g., Red-tailed, Red-shouldered, and Swainson's), woodpeckers (Downy and Nuttall's), and certain warblers are heavy users of wooded riparian areas and are largely eliminated from this community by the absence of an upper tree stratum. On the other hand, there is still sufficient vegetative diversity and cover to provide useful habitat for many common species of birds and mammals.
Other variables besides species composition influence wildlife use of this community; continuity (the linear distribution) is often interrupted at property lines or by periodic or partial maintenance; adjacent habitats (agricultural, urban, aquatic) in part determine the distribution of species in and extent of their dependence on this modified riparian vegetation-type.
Brushy Riprap .—Brushy riprap, one of the more common conditions on Delta levees, is valuable as wildlife habitat where riprapped banks have not been disturbed by inspection or maintenance for several years, or where riprapping has been limited to lower portions of the levee, allowing natural vegetation to remain on the upper levee. Vegetation here is less varied than in scrub-brush. Common wild blackberries predominate; other species include occasional shrubby alders at about mean water line, stinging nettles, wild radish, willows, and smartweed. Only when the vegetative cover on riprapped banks has grown almost out of the brush stage does the habitat appear to support wildlife comparable to shrub-brush banks.
Riprap which is exposed at low tides precludes bank burrowing by beavers, muskrats, and kingfishers. Hawks, woodpeckers, most warblers, and flycatchers are eliminated from this kind of substrate; ground foragers and swallows predominate. The yellow-throat, common on Delta freshwater marshes, uses the blackberry jungles of this community. Unvegetated, exposed riprap banks actually enhance striped and black bass habitat by providing substrate for crayfish, an important food source for bass.
Herbaceous Banks .—Herbaceous banks are most common on riprap or on banks where regular maintenance precludes all but summer annuals such
as reed grass and giant reed (at the waterline), scouring rush, blackberry, and wild oats, brome, other introduced grasses, and a variable cover of thistles, poison hemlock, with occasional willow saplings. During the fall and winter, when cover is at a minimum, wildlife use is limited. Summer growth encourages use of the habitat by small seed-eating birds. The only bird species whose presence would qualify this as distinctively "riparian" are the Belted Kingfisher, which perches on small structures associated with the grassy banks, and the swallows flying low overhead. The most conspicuous inhabitant of herbaceous banks in spring and summer months is the ground squirrel, whose potentially destructive burrow colonies are actually favored by cutting and burning of vegetation to facilitate visual inspection of levees (John Muir Institute 1978 a,b).
Unvegetated Riprap .—Unvegetated riprap is similar to herbaceous banks in its absence of vegetative cover. At most, the habitat offers perches and resting areas for birds using the open water and nearby brushy areas.
Neither unvegetated riprap nor herbaceous banks can really be considered "riparian" except according to a broad topographic definition of the term. Yet the majority of levees lining Delta waterways are being managed to achieve this result (fig. 4). A variety of intermediate conditions exist throughout the Delta: old riprapped banks on which willow has formed a discontinuous cover; partially neglected levee faces on which alder saplings have sprung up at the waterline; miles of giant reed lining the water's edge on grass-covered levee banks; occasional trees left to stand alone because the surrounding bare levee slopes permit visual inspection of otherwise pernicious root systems; landscaped residential and recreational frontages, notably deficient (for habitat purposes) in ground or brush stratum or liana. Each of these provides a limited habitat resource, always enhanced by the proximity of water and offering in return at least a narrow, if discontinuous, margin of vegetation or substrate to complement the aquatic environment.
Figure 4.
The safest levee from the perspective of flood control managers
is free of vegetation and open to visual inspection. Habitat
resources are virtually eliminated.
Interior Riparian Vegetation
The most extensive remnants of native riparian woodland occur around the outer margins of the Delta, along those portions of natural floodplains which were either not leveed or not cleared for agricultural cultivation. In a few locations, woodlands extend well back from the rivers along with the alluvial soils on which they grow. Where alluvial soils still exist as narrow levees or margins bordering peat lands in the central Delta, riparian woodlands are minimal and display less deversity of species and structure. Small riparian woodlands occur in the interiors of small islands or on island tips which were all or partially leveed during dredging operations but never cultivated.
The Sacramento/San Joaquin Delta Wildlife Habitat Protection and Restoration Plan (California Department of Fish and Game 1980) did not limit its survey to more obvious manifestations of riparian vegetation on levees along waterways and on the periphery of the Delta. Even highly modified island interiors can reveal wildlife habitats with riparian characteristics. These include seasonally fallow or ruderal lands, dredge disposal areas, drainages and irrigation ditches, and seasonally or periodically flooded agricultural lands.
Ruderal Lands .—Ruderal lands are not widespread in the Delta, where most available land is cultivated. However, a few long-untended fields, abandoned homesites, and agricultural lands damaged by sandy sediments of past floods (e.g., a portion of Brannan Island following the 1972 flood) have been allowed to revert to an almost natural state and are characteristically early successional communities. Species such as sandbar willow, Goodding's willow, and arroyo willow and thick tangles of blackberry are evidence of sufficient year-round soil moisture to eventually support a more diverse riparian-like community behind the levees. Even narrow brushy margins between cultivated fields become important wildlife corridors or islands in the predominantly cultivated landscape.
Unfortunately, ruderal lands in the Delta are totally dependent on individual farming practices. These areas must presently be regarded as transitory, subject to burning, plowing, crop changes, and "clean" farming. They are heavily used by mammalian predators and raptorial birds such as White-tailed Kites and Short-eared Owls because they support large populations of prey species—rodents, reptiles, and ground birds.
Dredge disposal sites .—In the Delta these areas resemble ruderal lands in that they support early successional communities and are highly variable, depending on the underlying residual vegetation, depth of spoils, elevation, and recency and frequency of deposition. Vegetation varies particularly with the age of the spoils; it is not uncommon to find older areas supporting young willows and cottonwoods, as well as herbaceous and other woody species. As with ruderal lands, year-round soil moisture is probably sufficient to eventually sustain larger riparian species if disturbances were discontinued (fig. 5).
Figure 5.
Dredge material disposal sites in the Delta have
demonstrated the ability to support adventitious
riparian species, such as willow and cottonwood.
Drainage and irrigation ditches .—Drainage and irrigation ditches border and criss-cross agricultural fields and levees throughout the Delta. A few major ditches, such as the one which parallels 8-Mile Road northeast of Stockton, have well-established woody species such as willows and cottonwoods along their banks; others are little more than strips of tules and cattails with brushy margins. While these might be considered minimally "riparian", they are so numerous and widespread that they contribute a significant amount of aquatic and bankside wildlife habitat to the Delta, especially for common birds and amphibians.
The most useful agricultural wildlife habitats in the Delta, and the most "riparian-like", are fields of corn and grain stubble, flooded for several months during the winter for leaching or to create waterfowl habitat. These fields support far greater numbers of individuals (although fewer species) than do more typical riparian areas in the Delta, and closely resemble or at least substitute for ancestral wetland which, prior to reclamation, supported many thousands of the waterfowl wintering in the Delta and elsewhere in the Central Valley. Whistling Swan, Canada Goose, and Sandhill Crane in particular are dependent upon partially flooded fields for winter feeding and resting habitat.
Some fields are flooded briefly in late summer and early fall to control weeds and centipedes. These fields also provide temporary habitat for migrating geese, swans, ducks, and shorebirds. The number, area, and location of flooded fields in the Delta vary considerably from year to year, depending on weather and current leaching practices.
Constraints and Opportunities for Surrogate Riparian Vegetation in the Delta
Constraining Factors
The numerous technical and economic factors responsible for restricting vegetation on Delta levees have been the subject of study and debate for more than two decades and are discussed in many other reports (e.g., California Department of Fish and Game 1980; Whitlow etal . 1979). Briefly, uncontrolled vegetation on levees presents a potential hazard to levee stability. Trees with laterally spreading root systems, such as some willow species, provide paths for piping of water. On the other hand, trees with shallow root systems, such as alder, are subject to wind throw, taking large chunks of the levees with them when they fall. Dense foliage or undergrowth obscures the levee face from easy visual inspection, impedes emergency operations, and, in the opinion of some, attracts burrowing animals.
Opinions vary concerning the contribution of vegetation to erosion control below the high water line: tules, for example, provide a natural breakwater in some situations but cause erosive eddy currents in others. There is general agreement that rock revetment (riprap) is the most reliable means of stabilizing eroding or incompetent banks. However, there is disagreement on the required height of riprap, that is, whether it should be placed below the mean water line, up to the mean water line, or up to the flood stage. The height of the riprap is critical to the amount of vegetation that can be supported on the waterface of the levee.
Riprap is an expensive treatment; its absence on older substandard levees which support some vegetation is often an indication not of preference for vegetation but of lack of sufficient funds or neglect. In all cases, economics are a major determinant in levee construction and maintenance methods. Riparian vegetation is viewed as a costly indulgence both to introduce following construction and to manage as part of ongoing levee maintenance, especially in a region in which agriculture is central to the economy and is dependent on a largely private levee system to separate land from water.
Opportunities
We have applied the term "surrogate" in this discussion to distinguish native riparian vegetation from variant forms of riparian vegetation and wildlife habitat that occur in the Delta. The term also reinforces the notion that protection of existing stands of riparian vegetation and reintroduction of riparian-like vegetation on or off levees under the evident constraints will involve some sacrifices and compromises.
At the outset, protection of existing riparian vegetation will require specific preservation programs that, through acquisition, easement, or other means, assure protection and management in perpetuity. A few of these areas are already publicly owned or have been earmarked for preservation. Certainly, all extant stands of riparian woodland in the Delta have been identified (US Army Corps of Engineers 1979; California Department of Fish and Game 1980). Encouraging expansion of riparian woodland (forest) would require identification of appropriate locations for reintroduction of species and protection through similar preservation programs.
Reintroduction or adoption of more sensitive maintenance practices of riparian-like vegetation will entail either acquisition, easements, subsidies, cost sharing, or a high degree of voluntary cooperation on private levees or in island interiors, to be coupled with one or more of the following management approaches.
Management and "Landscaping" of Permitted or Introduced Vegetation
Bulletin 192 titled "Plan for Improvement of Delta Levees" (California Department of Water Resources 1975) suggests that trees, shrubs, and grasses could be planted on levees between the top of riprap (1.5 ft. (0.5 m.) above mean high water level) and the crown (on a levee with adequate cross-section). Bulletin 192 also recommends that "desirable trees such as oak and crepe myrtle should be retained . . .(that there be) selective clearing of dead, diseased, and unwanted types of vegetatation . . . ." (ibid .). The California Reclamation Board also recommends selecting tree species for maximum height (no more than 40 ft. (12 m.)), deep roots and clean trunks, deciduous foliage; placement of trees above the design floodplain, spacing (8 m. on centers), and limiting numbers (California State Reclamation Board 1976).
These recommendations, if followed throughout the Delta, would result in: 1) elimination of important and characteristic riparian vegetation strata, principally groundcover, brush, and liana; and 2) elimination of certain native species whose growth habit or other characteristics are incompatible with levee design specifications, and possible substitution of other non-native species according to specifications.
However, viewed positively, and notwithstanding the costs, a managed levee riparian community is better than none. Careful planning of revegetation projects and selective management can produce open, not brushy, growth; vegetation at the water's edge; topping of trees at maturity and removal of dead material; selection of species for non-pernicious root systems; and selective eradication of weedy or intrusive exotics.
Overconstruction of Levees
Where technically and economically feasible, overconstruction of levees to accommodate root zones of potentially damaging trees or construction of setback levees to recreate floodplains can be an ideal management practice to allow for growth of riparian vegetation. There is no question among engineers that, in the absence of structural armor (revetment), an overconstructed levee with berm is the only condition under which riparian vegetation can be fully tolerated within the floodplain of Delta waterways. There is sufficient evidence in limited situations to demonstrate that riparian vegetation under these circumstances aids in dissipating the energy of floodflows and waves against the main levee. The circumstances also permit a riparian forest that can safely reach the full maturity of an 18-m. (60-ft.) or higher upper canopy and favor the dense growth of vines and shrubs that are so important in niche differentiation.
Relocation of Riparian Vegetation
Riparian vegetation may be relocated off levees by creating suitable physiographic and hydrographic conditions not necessarily associated with the levees, at least not on their waterside. Many opportunities exist for permitting and encouraging riparian-like vegetation in the Delta so as not to compromise levee stability: on the landface of levees, in untended or uncultivated portions of agricultural lands, along drainage ditches, and on dredge material deposits. Vegetation even on the landface of levees will require a certain amount of landscaping to exclude "uncontrolled growth" and root systems. Ruderal lands, though limited in extent in the Delta, permit the "uncontrolled growth" that often provides the most diverse habitat niches. Their availability, however, is tenuous, varying from year to year.
Carefully managed dredge materials probably offer one of the best opportunities for partial riparian regrowth, because areas are acquired and deliberately set aside for that purpose. In the past, indiscriminate placement of spoils has eliminated or damaged valuable habitat. With proper location, timing, and management of spoils, new wetlands and riparian conditions can be simulated in island interiors, converting a liability to an asset.
Literature Cited
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