Hydrology of the Central Valley

There is significant hydrologic diversity throughout the Central Valley, and it was this diversity which was in part responsible for differences between individual riparian forests. For example, the Valley has two major riverine hydrologic systems: that of the Sacramento Valley component in the north and of the San Joaquin Valley component in the south. The influences of these major hydrologic systems on the nature of

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

[2] Edwin F. Katibah is an Associate Specialist, Department of Forestry and Resource Management, University of California, Berkeley.

the riparian forests associated with them were profound. Figure 1 depicts the Central Valley and its major surface hydrology as it may have appeared under pre-settlement conditions.

Figure l.
Surface hydrology of the Central Valley as it may have appeared
around 1850. Areas in black within the Tulare Subbasin represent
seasonal lakes. Shaded areas, shown throughout the Valley,
indicate flood basins and freshwater marshes.

Hydrology of the Sacramento Valley

The Sacramento Valley is bordered by the mountains of the Coast Ranges to the west, the Klamath and Cascade Ranges to the north, and the Sierra Nevada to the east. To the south, the Sacramento Valley joins the San Joaquin Valley at the Sacramento/San Joaquin River Delta. The comparatively dry interior Coast Range mountains have no large rivers draining into the Valley, only streams, some of the larger being Stony, Cache, and Putah Creeks. The Sacramento River originates in the Klamath Mountains and is joined by two rivers, the McCloud and the Pit, in what is now Shasta Lake. The Sierra Nevada mountains to the east provide the greatest number of rivers and major streams draining into the Sacramento Valley—the Feather, Yuba, Bear, and American Rivers, and Butte and Big Chico Creeks.

Numerous other streams also flowed into the Sacramento Valley from the surrounding mountains. Not all of these streams actually reached the Sacramento River. Historically, natural levees and naturally occurring flood basins prevented some streams from reaching the main rivers. Instead, these streams spread out "through a welter of distributaries" (Thompson 1961) on the Valley floor. These distributaries typically ended in "sinks" of tule marsh. Putah, Cache, and Butte Creeks are among those streams which never joined the main river network in the Sacramento Valley.

The Sacramento Valley and its surrounding foothills, unlike the San Joaquin Valley region, receive substantial rainfall in the winter and early spring. This resulted in Sacramento Valley rivers experiencing maximum flows from December through March instead of May and June as is characteristic of most western rivers, including those in the San Joaquin Valley (Fortier 1909). Snowmelt fortified the river flow in the Sacramento Valley through the late spring. Annual summer drought brought the low flow rates found in these rivers through late fall.

During the peak flows of the Sacramento Valley rivers, the flood basins were filled by sediment-carrying waters. The natural levees dividing the flood basins from the major rivers were initially developed and then augmented by this annual flood cycle. Impressive natural levees along the Sacramento River, ". . . from 5 to 20 feet above the flood basins . . ." and 1.6–16 km. (1–10 mi.) in width, averaging 4.8 km. (3 mi.), ". . . formed corridors of generally dry land during times of flooding . . ." (Thompson 1961). The other major Sacramento Valley rivers and streams also formed well-developed natural levees.

Hydrology of the San Joaquin Valley

The San Joaquin Valley is bounded by the flat relief of the Sacramento/San Joaquin River Delta to the north, the mountains of the Sierra Nevada to the east, the Coast Ranges to the west, and the Tehachapi Mountains to the south.

The Coast Ranges and the Tehachapi Mountains bordering the San Joaquin Valley are very arid. Thus, the streams which originate from these mountains were characteristically intermittent in flow. Probably the most notable of these intermittent streams was Los Gatos Creek, whose alluvial fan helped form the Tulare Subbasin, a major influence in the hydrology of the San Joaquin Valley.

Numerous Sierra Nevada rivers and streams flowed into the San Joaquin Valley, including the Cosumnes, Mokelumne, Calaveras, Stanislaus, Tuolumne, Merced, Chowchilla, Fresno, San Joaquin, Kings, Kaweah, Tule, White and Kern Rivers.

The San Joaquin Valley is itself divided into two distinct hydrologic subbasins: the San Joaquin and the Tulare. The San Joaquin Subbasin is drained by the San Joaquin River; the Tulare Subbasin has no perennial surface outlet.

The Tulare Subbasin was formed at the south end of the San Joaquin Valley by the merging of alluvial fans from the Kings River to the east and Los Gatos Creek to the west (Cone 1911). Water originating from the major Tulare Subbasin rivers—the Kings, Kaweah, Tule, White, and Kern—flowed into this subbasin and found no normal outlet to the sea. Instead, large inland lakes formed—the Tulare, Buena Vista, Kern, and Goose. These largely temporary lakes, extremely shallow as they flooded the nearly flat landscape, rose dramatically as winter and spring runoff filled them. As the seasonal lakes filled beyond capacity they flowed into one another, finally rising above the natural alluvial barriers which divided the Tulare and San Joaquin Subbasins, sending tremendous quantities of water down the Fresno Slough into the San Joaquin River.

Later in the season, after the overland flow of water had ceased, substantial quantities of water were still drained from the Tulare Subbasin into the San Joaquin River via subsurface flow. This underground accession may have doubled the San Joaquin River's volume (Irrigation in California 1873). This undoubtedly helped to maintain the flow of the San Joaquin River in its southern reaches during the long, dry California summers.

The San Joaquin Valley rivers, whose waters were primarily snowmelt, tended to reach maximum flow in May and June. In contrast, peak flow of the Sacramento was usually in March, although some of the major peak flow rainfloods have occurred much earlier in the winter (1955–56 flood—December and January; 1964–65 flood—December and January; 1970 flood—January). In addition, the San Joaquin River's flow into the Delta in its peak flow period was less than onehalf the discharge rate of the Sacramento River during its usual peak flow period in March. Despite this difference in peak flow timing, the two rivers discharged approximately equal amounts of water into the Delta.

San Joaquin Valley rivers and streams in some instances did not produce the large, natural levees characteristic of the Sacramento Valley. Peak water flows in San Joaquin Valley rivers and streams were typically less than those in the Sacramento Valley, thus limiting their ability to pick up and carry sediment for great distances. Natural levees did form along the major northern San Joaquin Valley rivers—the Tuolumne, Stanislaus, Merced, Mokelumne, Cosumnes, and northern San Joaquin.

The southern (upper) reaches of the San Joaquin River developed natural levees only poorly, and only as the river entered the Valley floor. Never a particularly big river, it ranked third in peak flow after the Tuolumne and Kings Rivers (Cone 1911). Relatively low-energy peak flows resulted in suspended sediment deposition and natural levee formation only where it first entered the Valley. From there until it reached Fresno Slough, the San Joaquin River received no surface tributaries. At that point it received the surface floodwater flows through the Fresno Slough from the Tulare Subbasin and the underground flow through the extensive Tulare Subbasin aquifer.

Both of these flows were substantial, but both lacked significant sediment content. The overland flow through Fresno Slough had already deposited its sediment load in the shallow Tulare Subbasin lakes. The subsurface waters had been filtered of any sediment long before they joined the San Joaquin River. Thus while the southern San Joaquin River gained a large water accession, especially during the peak spring flood, it was unable to build any significant natural levees because of the low sediment load. With no natural levees to contain its waters, the San Joaquin River spread out over the flat Valley floor, sustaining the large freshwater marshes still found there today. The first major sediment-carrying waters to reach the San Joaquin River for many miles occurred at its confluence with the Merced River. From here to the Delta, substantial natural levees were built along the San Joaquin River.

The Tulare Subbasin rivers developed natural levees where these rivers first entered the Valley. The shifting courses of these rivers undoubtedly allowed many miles of levees to be formed, though they were quite narrow and confined compared to the levees of the Sacramento Valley rivers.