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

Hydrology

Instream Flows

In the Boise-Payette-Weiser subbasins, the IDFG recommended a number of instream flow regimes. Through its state water basin planning process, the Idaho Department of Water Resources also recommended instream flows in the Boise and Payette subbasins. To date, instream flows have been licensed in the Boise and Payette River subbasins (Table 10).

There are no licensed instream flow water rights in the Weiser River subbasin. The IDFG also recommended a number of instream flow regimes in the three subbasins that have never been acted upon by the IWRB (Table 11). These prior recommendations by the IDFG were intended as short-term minimum subsistence flows and were not meant to be long-term biologically based flows necessary to sustain aquatic resources.

The BOR developed a set of recommended reservoir pool elevations and flows for fisheries resources for the upper Snake River Basin as part of the needs assessment for the Snake River Resources Review (BOR 1998). The BOR used technical work groups comprised of experts from agencies, industry, tribes, and academic institutions to develop recommendations. Recommendations for the three subbasins are illustrated in Table 12. The Weiser River subbasin was not part of this review.

Boise River Subbasin

Stream hydrographs peak from late March to May because of snowmelt runoff. The runoff varies with south facing aspects at elevations less than 4,500 feet, warming early with resulting peak runoffs occurring as early as late March. High elevation lands with deeper snowpacks generate peak runoff beginning in late April and last until late May. Rain falling on snow in winter and spring cause rapid increases in stream flows. These rain-on-snow events usually occur in the elevation band between 4,500 and 5,000 feet. The peak runoff periods are followed by warm, dry summers, which greatly decrease stream flows. Seeps and springs provide perennial flows to streams in higher elevations, and smaller streams in the lower elevations tend to become dry before the end of summer. Periodic localized summer thunderstorms can result in flash floods within small drainages. The fall climate reduces transpiration in plants, and additional ground water results in slight increases in stream flows.

The stream flow regimes in the Upper Boise River watershed have been dramatically altered from historical conditions. Anderson Ranch Dam and Arrowrock Dam isolate migrant fish populations in the subbasin. Downstream dams on the Snake and Columbia River systems have blocked anadromous fish passage. Remaining migrant fish species have adapted from a fluvial existence to an adfluvial and fluvial lifestyle, wintering in reservoirs. Only the South Fork Boise River below Anderson Ranch Reservoir has had major stream flow alteration. In low water years the discharge from Anderson Ranch Dam is regulated for irrigation (1,700 cfs), intermediate (600 cfs), and base flow conditions (300 cfs).

The Payette River Subbasin

Nothing reported.

The Weiser River Subbasin

Nothing reported.

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