Photo: Stuart Allen's Lake Berryessa Line

The Putah-Cache Bioregion Project:

Natural History and Landscape

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Location
Climate
Geology
Water
Flora and Fauna
Further Reading

Location

The Putah-Cache Bioregion is a sub-region of California's Central Valley Bioregion. This sub-region begins just west of Sacramento, extending into the California Coast Range, and encompasses roughly 2500 square miles. Three watersheds make it up: Cache Creek, beginning with feeder streams north of Clear Lake in the Coast Range, flowing through the Capay Valley, and through Woodland in the Central Valley; Putah Creek, which begins at Cobb Mountain in the Coast Range, flows into Lake Berryessa, makes its way through the Monticello Dam outflow, and thence into Winters and Davis in the Central Valley; and Willow Slough, which drains the Rocky Ridge on the eastern edge of the Coast Range as well as the area of the Central Valley between Cache Creek and Putah Creek. All the watersheds now end in the Yolo Bypass, an overflow channel for the Sacramento River.

Historically all three watersheds drained into the Sacramento River in winter, so that forms its eastern boundary. The upper watersheds, in the Coast Range, are clearly divided from other watersheds by the mountain ridges. However, the watershed boundaries blur in the Central Valley. Relief is very low in the valley, so there are no clear divides. Also, the creeks and sloughs overflow during regular winter floods, and recharge the groundwater during summer. Thus the watershed area is not necessarily a question of what drains into the creeks, but what flows out of the creeks.

Climate

The entire watershed has a Mediterranean climate, temperate and rainy in winter, hot and dry in summer. The upper part of the watershed includes mountains high enough to get snow, while the lower, Central Valley part, has night frosts only a few times a year. Summer highs regularly rise above 110 F. Because the Central Valley is narrow, inversion layers are common, trapping fog in winter and smog in summer. The mountains are rather more airy.

California weather is notably unstable, with regular periods of drought and flood; a "normal" series of years in the valley runs something like this: 12"; 25"; 18"; 22"; 15". In recent years (1994-1998) winter floods have occurred annually, but seven years of drought preceded them. The Coast Range gets about 40-60 in. of rain yearly, while Davis in the Central Valley averages about 17 in. per year.

Geology

The geology of the bioregion varies from volcanic areas near Clear Lake, through seafloor sandstone at the eastern edge of the Coast Range, to the miles-thick alluvial deposits of the Central Valley.

The area near Clear Lake includes several currently inactive volcanoes, notably Mt. Konocti. Hot springs abound there, and a geothermal plant under Cobb Mountain makes use of the heat underlying the area.

The rest of the Coast Range portion of the bioregion consists of Franciscan melange (bands of muddy sandstone and chert, interleaved with basalt, and massively deformed), serpentine, and Great Valley Sequence seafloor (similar to Franciscan melange in age and composition but with much less deformation).

Associated with the volcanic areas and serpentine belts are mineral deposits such as mercury, sulphur, silver, and some gold.

The lower bioregion, in the Central Valley, rests on alluvial deposits that are literally miles thick. The Central Valley used to be an arm of the sea, but deposits from the Coast Range and Sierras gradually filled it in. The westernmost part of the lower bioregion, nearest the Coast Range, is somewhat rolling, but the area flattens until it is nearly plane at the Yolo Bypass. Because of the alluvium, gravel is mined around (and, until recently, in) Cache Creek.

Water

Though water is relatively abundant in the upper bioregion, the Central Valley section uses more water than it normally receives. Irrigation water comes largely from Putah and Cache Creeks, with the sloughs carrying off the tailwater from irrigated fields. Cache Creek Diversion Dam supplies irrigation water for Yolo County, while the Putah Creek diversion supplies water mainly to Solano County, outside the historic watershed.

Most drinking water comes from groundwater. Some irrigation water is also pumped from groundwater.

One lake in the bioregion, Clear Lake, is natural, the result of a landslide that dammed its original outlet. The other large lake in the bioregion, Lake Berryessa, is a reservoir created when Putah Creek was dammed in 1957. Virtually all other lakes in the region are also reservoirs.

Water quality and abundance are issues in the region. In the upper watershed, mercury leaches into the water. In the lower watershed, pesticide residues have been found in well water. The water itself is hard. Due to groundwater pumping, subsidence is a problem in some areas, particularly near Woodland. Creek fisheries have been affected in drought years because of water being diverted from the creeks.

Flora and Fauna

In the upper bioregion, underlying serpentine belts produce serpentine plant communities, including some rare plants. Redwoods grow in the upper bioregion near Cobb Mountain; elsewhere in the upper bioregion, chapparal, mixed oak and manzanita forests, and oak woodlands are common.

In the Central Valley, virtually all land is urban or agricultural, and little remains of the original plant communities. Some riparian corridors survive, with oak, willow, walnut, and cottonwood trees. Tule and cattails are common in the sloughs. Native plants that once made up the valley prairie, such as California poppy, lupine, and owl clover, appear by roadsides.

The upper bioregion includes "charismatic mega-fauna" such as tule elk (originally valley fauna), black bears, mountain lions, coyotes, bobcats, and eagles, both bald and golden.

The lower bioregion is part of the Pacific Flyway. Thus there are abundant hawks and waterfowl during the fall and spring migrations. Many hawks and waterfowl winter here, and some live here year round. Egrets and blue herons are common. The rare Swainson's hawk nests here.

Further Reading

A short list. The Barbour book includes an extensive bibliography.

  • Alt, David D. and Hyndman, Donald W. 1975. Roadside Geology of Northern California. Missoula: Mountain Press.
  • Barbour, Michael, et al. 1993. California's Changing Landscapes: Diversity and Conservation of California Vegetation. Sacramento: California Native Plant Society.
  • Jepson, Willis L. 1993. The Jepson Manual: Higher Plants of California. James C. Hickman, Ed. Berkeley: University of California Press.

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