Editor Stacey H. Stovall, Conservation Innovations, Inc. Subbasin Team Leader

Drainage Area

The Payette River subbasin is located in southwestern and west central Idaho and is approximately 3,300 mi². This subbasin drains portions of Adams, Boise, Valley, and Washington Counties. Major cities and towns include McCall and Cascade. Important streams include Big Willow Creek, Deadwood River, Fortynine Slough, Gold Fork River, Kennally Creek, Lake Fork, Little Willow Creek, North Fork Payette River, mainstem Payette River, South Fork Payette River, and Squaw Creek. Approximately 260 named lakes and reservoirs occur in the subbasin. The larger reservoirs include Cascade, Deadwood, and Paddock Valley reservoirs. Larger natural lakes include Box, Granite, Little Payette, Payette, and Upper Payette lakes.

The Weiser subbasin is located in southwestern Idaho and is approximately 1,500 mi². This subbasin drains portions of Adams and Washington Counties. Major cities and towns include Council and Cambridge. Important streams include Crane Creek, Hornet Creek, Keithly Creek, Little Weiser River, Monroe Creek, North Crane Creek, Pine Creek, South Fork Crane Creek, and the mainstem Weiser River. The subbasin contains ten reservoirs and one named lake. The largest reservoir is Crane Creek, and Rush Lake is the only natural lake in this subbasin.

Topography/geomorphology

Columbia River basalts are prominent in the Weiser and Payette River subbasins. The southeastern extent of the massive Columbia River basalt flows occur within the Payette River watershed, extending well east of Horseshoe Bend, Idaho. Quaternary alluvial surficial deposits are extensive in the Lower Boise River, North Fork Payette River, and mainstem Payette River watersheds. The Lower Boise River, the Payette River, and to a lesser extent the South Fork Boise River watersheds extend south into geologic features that are more prominent on the Snake River Plain: Pliocene stream and lake deposits and Pleistocene to Pliocene basalts and associated tuffs and volcanic detritus.

The Cretaceous Idaho Batholith and younger Tertiary granitic intrusions dominate several watersheds of the Boise-Payette-Weiser subbasins. Parent rocks of the formation are composed of biotite granodiorite, a medium-grained intrusive igneous rock that disaggregates easily, especially on the steep slopes of the subbasins.

Topographical relief of the subbasins is reflective of a terrain that once attained a mature erosional level (by the Middle Tertiary) and subsequently uplifted thus re-initiating stream erosional processes. Quarternary glaciation occurred primarily on isolated high elevation peaks. Alpine glacier systems formed in the western Salmon River and Boise Mountains. Though large scale glacially derived physiographic features such as broad U-shaped valleys are mostly absent, localized evidence of alpine glaciation (pothole lake systems and glacial cirques) is common in the subbasins on upper slope and ridge top positions of higher elevation ridge systems. Stream erosion has played the predominant role in shaping the physiography of the subbasins. In the mountainous portions of the subbasins, stream erosion since the Middle Tertiary has given rise to topography characterized by relatively narrow, V-shaped valleys, steep valley side slopes, and relatively narrow ridge systems.

The combination of the physical characteristic of Idaho Batholith biotite granodiorite and the physiographic nature of the landforms within these subbasins give rise to high natural potential for erosion. The Boise-Mores Creek, Middle Fork Payette River, North Fork Payette River, North and Middle Fork Boise River, South Fork Boise River, and South Fork Payette River watersheds are particularly subject to rapid erosion and mass wasting. In areas with intense land management activities, erosion rates are even higher.

The coupling of high susceptibility to erosion and land management practices such as timber harvest activities, road construction, livestock grazing, and natural fire disturbance events has given rise to recent extreme mass wasting within affected subbasin watersheds. For example, in 1994, a major wildfire occurred in Trapper Creek, a logged and heavily roaded tributary of the North Fork Boise River. In late summer 1995, Trapper Creek suffered a debris flow that removed most of the streambed, all vegetation, and aquatic biota from the creek. The debris torrent made up of predominantly fine sediment, coarse rock, and plant material was deposited into the North Fork Boise River.