The Central Valley Riparian Mapping Project^[1]

Charles W. Nelson and James R. Nelson^[2]

Abstract.—The Central Valley Riparian Mapping Project was initiated in 1978 by the California Department of Fish and Game. Maps showing the occurrence of riparian vegetation on the depositional flatland or floor of the Central Valley were compiled from existing aerial photographs (35mm. color slides and high altitude, false-color infrared U-2). Special techniques devised to map from 35mm. color slides are described. Vegetation-types were divided into six riparian categories, two subcategories, and two nonriparian categories. Modifying characters were used to indicate unusual circumstances under which riparian vegetation occurred. These maps provide a record of this diminishing resource on the floor of the Central Valley as it appeared in 1976. This data base will permit the documentation of changes in this important riparian resource in the future.

Introduction

Project Background

The Central Valley Riparian Mapping Project (CVRMP) was the first attempt by the State of California to map, quantify, and monitor the distribution of riparian resources in the Central Valley. The distribution of riparian vegetation has been described by previous investigators using literature searches, early soil surveys, and personal communications (Conrad etal . 1977; McGill 1975; Roberts etal . 1977; Thompson 1961). Detailed mapping of riparian vegetation on the Sacramento River was recently completed by the California Department of Water Resources (DWR) (1978) and contributed significantly to the information on that riparian system. Work by the USDI Fish and Wildlife Service (FWS) (Cowardin etal . 1977) will provide a complete inventory of riparian vegetation in the Central Valley, but at a smaller scale. The CVRMP provided large-scale maps which record the distribution of riparian vegetation and provide a tool for quantifying the amount of this resource remaining in the Central Valley.

Legislation enacted in 1978 (AB 3147, Fazio) appropriated funding to the Department of Fish and Game (DFG) for a study of the riparian resource of California's Central Valley up to the upper edge of the digger pine/blue oak zone (Küchler 1977)—approximately 760-m. (2,500-ft.) elevational level—and the South Lahontan and Colorado Basins of the California Desert. This legislation was intended as a first step toward acquiring the data needed to properly care for this important and diminished resource.

The CVRMP was an element of the DFG's overall riparian study program and was designed to contribute to development of study recommendations for protection of the resource. In addition, the maps provide a record of the resource as it existed in 1976 (the average date of the aerial photographs). From this data base it will be possible to determine change in the riparian resource in the years to come. The maps can also be used to determine and analyze potential impacts of development upon the resource on a day-to-day basis.

The CVRMP was carried out by two mapping teams comprised of graduate and undergraduate students from California State University, Chico, and California State University, Fresno. While other mapping projects have mapped portions of the Valley's riparian resource, this is the only study that provides complete coverage of the entire Valley floor.

Project Objectives and Scope

The CVRMP was intended to provide a baseline assessment of existing riparian vegetation-types through their categorization and mapping. This study documented the extent and distribution of this resource and provided a basis for quantifying existing riparian vegetation (see Katibah, Nedeff, and Dummer 1981). The results of the

[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].

[2] Charles W. Nelson is Cartographic Technician/Lecturer in Geography at California State University, Chico. James R. Nelson is a Botanist with the California Energy Commission, Sacramento.

study have been incorporated into the DFG's fish and wildlife planning effort and are being used to direct riparian studies as well as preservation and restoration programs.

Methods

Aerial photos depicting the Central Valley depositional flatland were used to produce maps of riparian vegetation categories and their spatial distribution. Methods were devised which allowed use of existing aerial photography in an accurate and rapid fashion at the lowest possible cost. A technique was devised to expedite the mapping task using available 35mm. photos. Project team members were trained in the recognition, interpretation, and mapping of riparian vegetation categories.

Imagery

Aerial photography served as the data base for the mapping process. Positive transparencies (35mm. color slides) taken by the DWR over a five-year period (average date 1976) were used as the primary information source for the project. These photos were taken at low altitude (ca. 1,525 m. (5,000 ft.)) and provided coverage for most of the study area. The large scale of this true color (Kodachrome) imagery made interpretation of vegetation-types relatively simple. The 35mm. format was easy to handle during the information transfer process using techniques described below.

For those areas not covered by DWR photography, high altitude, U-2, false-color infrared photography was used. Further coverage was provided by standard panchromatic (black-and-white) 9 × 9-in. photographs.

Interpretation

Interpretation of vegetation categories from aerial photographs was accomplished through careful evaluation of standard image characteristics (i.e., color, pattern, shape, association, size, shadow, and topographic location). A detailed description of the vegetation categorization appears below.

An initial field reconnaissance was conducted to familiarize all team members with the riparian vegetation categories. Discussions were held periodically during the mapping phase to ensure the correct interpretation of unusual photo-signatures and check the accuracy of completed maps. This routine procedure kept all mappers aware of various interpretive and cartographic problems and assured greater accuracy and consistency in the mapping effort. Ground-truth checks were conducted on most maps to further ensure map accuracy.

Data Transfer

The mapping of vegetation over large areas required the development of a system that would allow quick, accurate scale adjustment and adequate illumination of the image onto the base map. For this purpose, a transfer system was devised for the DWR 35mm. color slides.

Information from the 35mm. slides was traced onto mylar or blueline sheet overlays of 1:24,000 USDI Geological Survey (GS) quadrangle maps (quads) through the use of a system which projected the photo-image to the bottom of the map through a glass table. Figure 1 is a schematic diagram of this system. A Kodak Ektographic, high light-intensity slide projector (Model AF-2) with a 3-in. (f:3.5) close-up lens, was used to project the slides. The image was reflected up through the glass tabletop by means of a mirror which was mounted at a 45° angle. Slides were reversed when placed in the projector to allow for the reversal of the reflected image. Scale was adjusted either by moving the mirror toward or away from the projector (major-scale adjustments) or through the use of the projector's focus mechanism (minor-scale adjustments).

Figure 1.
Schematic of the data-transfer system. The image can
be projected at the same scale as the base map,
making transfer work accurate and efficient.

Cartographic Representation

Riparian vegetation categories were represented by polygons outlining each vegetation area. Each vegetation category polygon was labelled with a letter or letter/number code. Where vegetation strips were too narrow to outline (such as along a narrow stream or canal), the vegetation was indicated by a single solid line. Letter codes were placed within the polygon boundaries when space permitted. Where space was limited, they were placed outside, with an arrow drawn to the center of the area. Where the vegetation was indicated by a single line, an arrow was drawn from the category symbol to that line. Where more than one vegetation category occurred on a narrow (single-line) strip, a short perpendicular line indicated the boundary. When a single line intercepted a polygon or another single line, a label was placed on each line segment. Figure 2 is an example of the riparian vegetation map product.

Figure 2.
A sample of the riparian vegetation
map product. Scale = 1:24,000.

Mapping Criteria

The following criteria were used as the basis for mapping.

1) Areas with native or "wild" (nonagricultural) riparian vegetation which could be outlined were always mapped.

2) All continuous natural streamcourses associated wholly or in part with riparian vegetation were mapped. Where a natural stream had discontinuous riparian vegetation sections which were separated by short (e.g., 3.2-km. (2-mi.)) sections of vegetation-types not normally mapped, the entire streamcourse was shown.

3) Canals, discontinuous streams, or wet areas which appeared (from the imagery) to be dependent upon an artificial water source and devoid of woody riparian vegetation were not mapped.

4) Agricultural and urban areas were mapped only when they appeared as islands surrounded by riparian vegetation.

Mapping Category System

The major category system used in the CVRMP was based on structural differences (physiognomy) of vegetation units which could be readily discerned from aerial photography. Vegetation units were divided into six riparian or riparian-associated categories, two subcategories (for minor types occurring as part of major categories), and two nonriparian categories (used where nonriparian lands were surrounded by riparian vegetation). In addition, three modifying characters were used to indicate special circumstances under which riparian vegetation occurs. A "hybrid" system was devised to describe those areas which appear to be a mixture of more than one riparian vegetation category. In such cases, the codes of the two most predominant vegetation categories were indicated.

Riparian Mapping Categories

R1—Large Woody Vegetation

Large woody vegetation refers to the older, well-established riparian forests which are represented by tall (over 12 m.) woody vegetation. In the Central Valley bottomlands these areas are usually dominated by Fremont cottonwood (Populus fremontii ), black walnut (Juglans hindsii ), western sycamore (Platanusracemosa ), Oregon ash (Fraxinuslatifolia ), and willow (Salix gooddingii var. gooddingii and other spp.). Accompanying these species is usually a dense understory of shrubs and vines; wild grape (Vitiscalifornica ), blackberry (Rubus spp.), and mugwort (Artemisia douglassiana ) are a few of these species. This vegetation category may cover large areas along broad undisturbed floodplains or very narrow (sometimes discontinuous) strips where human land-use practices have encroached upon the "wild" vegetation.

The R1 category may be discerned in true color aerial photography on the basis of distinct bright green color or mottled color combinations, the evident pattern of tree crowns, relative topographic location, and occasionally by the occurrence of tree shadows. The color of the R1 category is typically much lighter (at times almost yellow-green) than the R1v category (the only other tall-tree category). R1 often shows color or tonal mottling which results from the occurrence of numerous tree and shrub species. The crowns of individual large trees are usually evident when surrounded by different species. However, a dense even stand of large trees (usually tall cottonwoods) may appear homogeneous throughout; in such cases individual tree crowns are less discernible. However, the occurrence of long shadows or a comparison with associated vegetation usually is adequate to accurately identify the signature as R1.

R1v—Valley Oak Woodland

This subcategory of large woody vegetation (R1) refers to the Valley Oak woodland plant

community. These are mature stands of well-spaced valley oak (Quercuslobata ) without a well-developed woody understory. Valley grassland species dominate the areas between trees. This vegetation category is generally associated with high terrace portions of lower elevation Central Valley rivers.

Valley oak woodland may occur adjacent to other vegetation categories near streams or as discontinuous isolated patches away from streamcourses. Before extensive land clearing, these isolated patches would have been part of larger woodlands associated with the other riparian vegetation categories. R1v can usually be discerned from R1 and R2 categories (large and low woody vegetation, respectively) on the basis of its dark green color. In addition, rounded, well-separated crowns often can be identified in older stands. As valley oaks are large, stately trees, shadows are also a good indicator.

R2—Low Woody Vegetation

This category represents an early successional stage of riparian forest development. Trees are younger, shorter (up to 12 m.), and may occur with shrub species. Willows and young cottonwoods usually dominate, although brush species occur in some areas.

Interpretation characteristics include nearly consistent coloration, an even photographic texture, and association with other vegetation categories. Low woody vegetation is generally light-green or gray-green in color. It usually appears as a consistently dense, closely spaced stand, although spottiness may occur. As an early successional type, this vegetation category can be expected to occur along sandbars, receding oxbow lakes and sloughs, and in disturbed areas such as canals and levees. It sometimes appears as an intermediate between open water (or sandbars) and the taller riparian forests. Also, it may occur alone, expecially along smaller streams in the lower foothills.

R3—Herbaceous Vegetation (Valley Grasslands)

Valley grasslands includes low (usually less than 1 m.), introduced and native herbaceous species which are mostly annual, although there are some perennial species. Occurrence may be natural (e.g., Valley Grassland plant community or perennially green herbaceous areas along streams) or the result of severe disturbance (constituting an early successional stage).

Two types of riparian-associated herbaceous vegetation were found in the Central Valley. Valley grasslands are treeless, low in stature, and brown during the summer months. In agricultural areas of the Central Valley they are almost exclusively located within a riparian corridor (i.e., uncultivated streamside lands) and may be surrounded by other kinds of riparian vegetation.

The other type of herbaceous riparian vegetation usually occurs along perennial streamcourses on Valley rangeland. It is low in stature, with little or no woody vegetation in evidence, but is green during the summer months, in contrast to the brown of the surrounding rangelands.

R3p—Perennial Seeps

This is a special subcategory of R3 referring to spring areas that are perennially green with herbaceous vegetation. This subcategory is not used for perennially green areas along streams and is differentiated by its patchiness and separation from streamside R3. Artificial seeps, such as those associated with irrigation canals, wells, or windmills, were not included in this subcategory.

M—Marsh

The marsh vegetation category includes intermittent or perennially wet areas with emergent herbaceous vegetation. These areas are characterized by dense stands of tall grass-like plants such as tules (Scirpus spp.), cattail (Typha spp.), sedges (Carex spp.), and rushes (Juncus spp.). These plants are found in, and sometimes interspersed with, continuously moist areas of mud, or standing or sluggishly-moving shallow water. Marsh areas are commonly associated with rivers, streams, lakes, canals, or depressions (sinks).

Marshes can be distinguished on aerial photographs on the basis of color, pattern, location, and association with other vegetation categories. On the photos, marsh appears as a mixed lightor dark-green color. The arrangement of marsh species varies from highly mixed stands (seen as mottled shades of green on the aerial photographs) to homogeneous bands around open pools (which appear as concentric rings around water areas). Marshes are usually found adjacent to or along canals, streams, sinks, and sloughs. They are often found adjacent to willows and herbaceous vegetation. Marsh occurring within channelized streams is often indiscernible. Where marsh occurs with taller mature forests, it is difficult to recognize except in instances where it covers very large areas. Since marsh vegetation was mapped only when it was found among or adjacent to other riparian vegetation categories, not all of the Central Valley marshland was mapped.

S—Sandbars and Gravelbars

Areas of sand and gravel or exposed rock are included in this mapping category. Vegetation is usually limited to very low willows, cottonwoods, and intermittent herbaceous growth undiscernible from the aerial photographs. Usually sandbars and gravelbars occur adjacent to a stream channel. Photographic signatures include white, gray, and brown colors.

W—Open Water

The open water classification includes standing or moving open waterways which are significantly free of vegetation. Sometimes these areas were difficult to interpret (especially where standing water was surrounded by tall, overhanging vegetation), as often this water displays a dull green color, flatness, and a very smooth, even texture, with occasional reflections evident on the aerial photographs. In other situations (particularly with moving water) the color may be darker. Where white areas occur, riffles or rapids may be present. Open water is usually apparent in stream and river channels. In areas where water cannot be seen on the photographs, even though it may be present, it was not mapped. Only water associated with riparian vegetation was mapped. For example, most manmade canals and reservoirs in the Central Valley were not mapped when devoid of significant riparian vegetation.

A—Agriculture

Agricultural lands partially or completely surrounded by riparian vegetation are included in this category. All cultivated and recently cleared lands are included. Agricultural areas which are adjacent to, but not surrounded by, riparian vegetation were not mapped.

U—Urban

The urban category includes those built-up areas which are completely surrounded by riparian vegetation. In practice, this mapping unit was seldom used. Any land cleared of its natural vegetation and put to industrial, commercial, or residential use would fall into this category.

Modifiers

The following modifying codes were used to signify special circumstances under which riparian vegetation might be found.

c—Channelized

This modifier was used where riparian vegetation exists along a watercourse which appears to have been modified by human activity to the point that natural stream contours are no longer visible.

d—Disturbed

Areas of severe man-caused soil disturbance were included within this modifier. Dredger tailings and gravel mining operations are the most common examples. Numerous ponds are found in some of these areas. Ridges of unvegetated gray rock occurring on the more recent sites indicate dredger tailings. At older locations, these ridges may be vegetated with a thin covering of herbaceous growth. Linear strips of R1 or R2 frequently occur between ridges. Most dredger tailings are ilustrated on GS 7.5 minute quads, confirming suspected identifications. Gravel mining may be identified by the presence of vegetated or unvegetated ponds or pits, especially if they have an unnatural shape.

i—Intermittent

Intermittent was used to designate spottiness or nonconsistent occurrence of a given vegetation category. When used with a single code symbol, or both code symbols of a hybrid notation, the interspaced areas should be interpreted as either S, W, and/or R3.

Category Hybrids

Where any area of vegetation could not be classified clearly as one of the major categories or subcategories, a "hybrid" of two codes was used. This was intended to allow for the most accurate representation of areas which have a mix of vegetation categories occurring in spaces too small to map individually. The hybrid system identified only the two most common vegetation categories, even though other types may be present.

The hybrid code itself consists of codes from the two most prominent categories, separated by a slash. The first of the codes represents the category which, on the basis of general appearance, seems to cover the greatest area. The other portion of the hybrid code covers the second largest portion of the outlined area. For example, an area of 45% R1, 35% R2, and 20% of any other vegetation was labeled R1/R2.

Where modifiers were needed, they were placed at the end of the hybrid code (e.g., R1/R2c indicates a canal which is lined with mixed R1 and R2 vegetation). Where a modifier is used, it refers to both portions of the hybrid symbol (e.g., R1/R2ic would indicate a channelized stream lined with intermittent, mixed woody vegetation).

Table 1 presents a summary of the vegetation codes, their descriptions, and the characteristics by which they were identified on the aerial photographs.

Literature Cited

Conrad, S.A., R.L. MacDonald, and R.F. Holland. 1977. Riparian vegetation and flora of the Sacramento Valley. p. 47–55. In : A. Sands (ed.). Riparian forests of California: their ecology and conservation. Institute of Ecology Pub. No. 15. 121 p. University of California, Davis.

Cowardin, L.M., V. Carter, F.C. Golet, and E.T. LaRae. 1977. Classification of wetlands and deep-water habitats of the United States (an operational draft). USDI Fish and Wildlife Service. Unpublished manuscript.

California Department of Water Resources. 1978. Sacramento River Environmental Atlas. California Resources Agency, Sacramento.

Katibah, E.F., N.E. Nedeff, and K.J. Dummer. 1983. Summary of riparian vegetation areal and linear extent measurements from the Central Valley Riparian Mapping Project. In : R.E. Warner and K.M. Hendrix (ed.). California Riparian Systems. [University of California, Davis, September 17–19, 1981]. University of California Press, Berkeley.

Küchler, A.W. 1977. Map of the natural vegetation of California. 1:1,000,000 + 31 p. A.W. Küchler. Department of Geography, University of Kansas, Lawrence.

McGill, R. 1975. Land use changes in the Sacramento River riparian zone, Redding to Colusa. 23 p. Resources Agency, Department of Water Resources, Sacramento, Calif.

Roberts, W.G., J.G. Howe, and J. Major. 1977. A survey of riparian forest flora and fauna in California. p. 3–19. In : A. Sands (ed.). Riparian forests of California: their ecology and conservation. Institute of Ecology Pub. No. 15. 121 p. University of California, Davis.

Thompson, K. 1961. Riparian forests of the Sacramento Valley, California. Annals of the Association of American Geographers 51:294–315.