Adapted from the 2002 Snake River Limiting Factors Report (Washington Conservation Commission)
The Middle Snake Watershed (WRIA 35) is located in the extreme southeast corner of Washington. The WRIA is bordered by the state of Oregon to the south, the state of Idaho to the east, the Palouse Watershed (WRIA 34) to the north, and the Walla Walla (WRIA 32) and Lower Snake (WRIA 33) Watersheds to the west. The basin drains approximately 2,250 square miles within the state of Washington. (NOTE - Approx. 340 square miles of the lower Grande Ronde Watershed is located within WRIA 35 boundaries but is not part of the planning effort). The Middle Snake Watershed encompasses portions of Asotin, Whitman, Garfield, and Columbia Counties within Washington. Diamond Peak, located in the headwaters of the Tucannon River, is the highest point in the basin with an elevation of 6,380 feet, while the confluence of the Snake and Tucannon Rivers is the lowest point at approximately 540 feet.
The total population of Asotin County in 2000 was 20,551, 19,256 of which lived in the cities of Asotin or Clarkston and surrounding areas. No major population centers are present in the Whitman County portion of the WRIA. The city of Pomeroy was the most populated area in Garfield County with 1,517 residents. The largest town in the Columbia County portion of the WRIA was Starbuck with a population of 130 in year 2000. Private land comprises 1,711 square miles (76%) of the WRIA, while the federal government manages 436 square miles (19%), and the state of Washington manages 103 square miles (~5%).
The Middle Snake Watershed is semi-arid. Average annual precipitation ranges from 5-10 inches in the lowlands along the Snake River up to 45 inches in the peaks of the Blue Mountains.
Primary surface water bodies include the Snake River, Tucannon River, Asotin Creek, and Pataha Creek. Little Goose Dam (RM70) and Lower Granite Dam (RM107) impound the Snake River in WRIA 35. The backwater from Lower Granite Dam extends upstream to the City of Asotin (RM146). The Snake River is free-flowing from this point up to Hells Canyon Dam (RM247) located upstream from the upper end of WRIA 35. Groundwater flows through cracks in the basalt layers as well as the porous sediments sandwiched between the basalts, carrying considerable quantities of groundwater that supply water for irrigation and municipal uses.
Historically, the Middle Snake Watershed was covered with prairie and canyon grasslands and shrub-steppe vegetation. Today, much of the land area in the WRIA (43%) has been converted to crop and livestock production. Non-irrigated row crops, primarily wheat, and grass-forb plant communities comprise roughly 37% and 30% of vegetative cover in the watershed respectively. Coniferous forests cover approx. 20%, while a mixture of shrubs and trees covers 7%.
Summer Steelhead have been observed throughout the watershed. Bull Trout have been observed in the North Fork Asotin Creek, Pataha Creek, the Snake River and the Tucannon River, and may be present in the South Fork Asotin Creek, Asotin Creek Mainstem, Charley Creek and George Creek. Spring Chinook have been observed in the Snake and Tucannon River and may be present in the North and South Forks of Asotin Creek and Alpowa Creek. Fall Chinook may be present in the Tucannon River. In addition, the Snake River mainstem contains populations of Sockeye and Sturgeon (for more detail, see Table 7 of the WRIA 35 Limiting Factors Analysis, available from the WA Conservation Commission).