 |
|
|
|
|
|
Ecoregion Description
125: Sacramento - San Joaquin
Major Habitat Type:
temperate coastal rivers
Author:
Text modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment. Island Press, Washington, DC, USA.
Countries:
United States
Boundaries:
This ecoregion lies almost entirely within the state of
Drainages flowing into:
The Sacramento and San Joaquin rivers join in the Central Valley to form an inland delta, which then drains the Suisun, San Pablo and San Francisco bays to the Pacific Ocean (Minckley et al. 1986).
Main rivers or other water bodies:
Within the ecoregion, the main freshwater systems are the Sacramento-San Joaquin rivers, the Pit River,
Topography:
The Central Valley is a large, flat valley that is bordered by the Sierra Nevada to the east and the Coast Ranges to the west.
Climate:
The ecoregion experiences a Mediterranean climate, with hot dry summers and cool wet winters. Temperature averages 13 to 19 oC, and precipitation ranges from 130 to 760 mm, with the north receiving more precipitation than the south (McNab & Avers 1994).
Freshwater habitats:
The confluence of the Sacramento and San Joaquin rivers form an inverted river delta that flows into San Francisco Bay, the largest estuary on the West Coast of North America. This area once supported enormous populations of migratory waterfowl in extensive freshwater marshes (Ricketts et al. 1999). Riparian woodlands were also once more extensive, and bordered many of the valley’s major rivers and tributaries. Vernal pool communities occur within the Central Valley in seasonally flooded depressions, including those that are saline or alkaline, terrace pools, and pools on volcanic soils. Several aquatic invertebrates are restricted to these habitats (Ricketts et al. 1999).
Terrestrial Habitats:
In addition to the high diversity of freshwater habitats, this ecoregion also once supported one of the most diverse, productive, and distinctive grasslands in temperate North America, including prairies, oak-grass savannas, and desert grasslands (Ricketts et al. 1999).
Fish Fauna:
About four to five million years ago, when the predecessors of the Sierra Nevada and Coast Ranges had been eroded down, fishes invaded the Sacramento-San Joaquin region from the ancient Columbia River system. Subsequent mountain-building isolated the fishes, and the Sacramento-San Joaquin became the center for evolution within the larger Central Valley region (Moyle 1976). Today, the Central Valley is one of the richest ecoregions in North America west of the Rockies in terms of fish species.
In total, the ecoregion supports nearly 40 native freshwater fish, including a number of anadromous species. Fish groups with high representation include anadromous lampreys, sturgeons, smelt, and salmonids, as well as cyprinids, and suckers. Additionally, there are the threespine stickleback (Gasterosteus aculeatus), Sacramento perch (Archoplites interruptus), tule perch (Hysterocarpus traski), and tidewater goby (Eucyclogobius newberryi). The Sacramento perch is the only centrarchid to occur west of the Rockies, and the tule perch is the only freshwater member of the surfperch family (Embiotocidae).
Description of endemic fishes:
Of the fish species endemic to the ecoregion, many have localized distributions. Species with historically small ranges include the extinct Clear Lake splittail (Pogonichthys ciscoides), and the rough sculpin (Cottus asperrimus). The primitive Sacramento perch was once widespread throughout the Sacramento-San Joaquin system, but today inhabits a much reduced range due to its inability to compete with introduced centrarchids (Moyle 1976). Other endemics include the hardhead (Mylopharodon conocephalus), Sacramento pikeminno (Ptychocheilus grandis), and splittail (Pogonichthys macrolepidotus).
Ecological phenomena:
The ecoregion is considered biologically outstanding because it contains the southernmost populations of five anadromous fish as well as four runs of Chinook (Abell et al. 2000).Evolutionary phenomena:
Like the neighboring Oregon and Northern California Coastal ecoregion, the Sacramento – San Joaquin ecoregion is noted for its higher taxonomic endemism in fish and amphibians (Abell et al. 2000). The region contains five endemic fish genera (Archoplites, Pogonichthys, Orthodon, Lavinia, Mylopharodon) and one near-endemic salamander genus (Hydromantes).
Justification for delineation:
Ecoregion boundaries are taken from Abell et al. (2000) and are based on subregions defined by Maxwell et al. (1995). Modifications to the boundaries of this ecoregion were based on a biogeographic assessment performed by The Nature Conservancy. The boundaries were modified to exclude the Surprise Valley watershed, now placed in the Oregon Lakes [124] ecoregion.
References/sources:
Abell, R., Olson, D., et al. (2000). "Freshwater ecoregions of North America" Washington, D.C.: Island Press.
Maxwell, J. R., Edwards, C. J., et al. (1995) "A hierarchical framework of aquatic ecological units in North America (Nearctic Zone)". St. Paul, MN. North Central Forest Experiment Station, USDA Forest Service.
Minckley, W.L., D. A. Hendrickson, and C. E. Bond (1986)"Geography of western North America fishes: description and relationships to intracontinental tectonism" In Hocutt, C.H.a.E.O.W. (Ed.). The zoogeography of North American freshwater fishes. New York: John Wiley & Sons.
Moyle, P. B. (1976). "Inland fishes of California" Berkeley: University of California Press.
Ricketts, Taylor H. Dinerstein Eric Olson David M. Loucks Colby J. (1999). "Terrestrial ecoregions of North America: A conservation assessment" Washington, D.C.: World Wildlife Fund.
