Section 303(d) of the Clean Water Act establishes requirements for states and tribes to identify and prioritize water bodies that are water quality limited (i.e., water bodies that do not meet water quality standards). States and tribes must periodically publish a priority list of impaired waters, currently every two years. For waters identified on this list, states and tribes must develop water quality improvement plans known as total maximum daily loads (TMDLs) that establish allowable pollutants loads set at levels to achieve water quality standards.

The hydrology of the river is complex, with numerous irrigation water withdrawal and return drains dominating both the flow and quality of the river. The presence of Black Canyon Dam has greatly altered the amount and type of sediment in the lower Payette River originating from the upper watershed.

Fisheries studies conducted by the Idaho Department of Fish and Game in 1997 indicate many of the same species supported by the river in 1974 were also supported in 1997. Mountain whitefish is the dominant cold water species. Warm water species can be found throughout the river, with non-game species being dominant.

Sources of pollutants include both point sources and nonpoint sources. Point sources are limited mainly to municipal wastewater treatment plants and confined animal feeding operations. Nonpoint sources are associated with agricultural, urban, suburban, and rural areas.

Nutrients have not been shown to cause impairment to the beneficial uses in this water body at this time. While total phosphorus and nitrogen are at concentrations that could cause nuisance aquatic vegetation growth, data show they do not. While dissolved oxygen concentrations do not drop below the water quality standard, monitoring indicated that aquatic growth is causing a fluctuation in dissolved oxygen levels. If it is determined that the lower Payette River is a significant source of nutrients to the lower Snake River (in the lower Snake River/Brownlee TMDL), reduction targets for the lower Payette River will be addressed at that time.

Summer water temperatures in the lower Payette River are warm and exceed water quality standards for both cold water biota and salmonid spawning. However, it was determined that other factors, including habitat modification and flow alteration, were also significant causes of impairment of beneficial uses. In addition, warm water temperatures that exceed water quality standards originate from Black Canyon Reservoir. Because of these conditions, a temperature TMDL was not developed. TMDLs were also not written for flow alteration and habitat modification because these are not pollutants as described under Section 303(d) of the Clean Water Act.

Fecal coliform bacteria levels exceed the water quality standards for both primary and secondary contact recreation. Increasing levels are noted from Black Canyon Dam to the Snake River, with an exceedance of the water quality standards from river mile 25 to the confluence. Overall, a fecal coliform reduction of 84% will be required to achieve water quality standards. The load allocation will focus on nonpoint sources only. The overall contribution to the fecal coliform bacteria load from point sources (municipal wastewater treatment plants) is 0.005%. If the total elimination of bacteria from the point sources were to occur, a total load reduction of only 0.07% would be achieved. Therefore, any reduction from point sources would not impact the overall load to the lower Payette River.

Big Willow Creek is a north side tributary to the Lower Payette River that enters the river between New Plymouth and Payette, Idaho. However, alterations made to Big Willow Creek now prevent the water body from discharging directly to the river.

Big Willow Creek was placed on the 1998 §303(d) list of impaired waters by EPA for reasons associated with temperature. It was subsequently carried forward to the 2002 Integrated Report (§303(d) list/§305(b) report). The information evaluated for this document confirmed temperature is impairing the beneficial uses of Big Willow Creek.

Effective shade targets were established for Big Willow Creek based on the concept that maximum shading under potential natural vegetation equals natural background temperature levels.

The perennial segments of Big Willow Creek were analyzed and lacked sufficient shade to meet targets.  A reduction in solar loading (direct sunlight) and an increase in shade is necessary for these segments to meet load objectives.

Much of the riparian community in some segments of Big Willow Creek was severely disturbed by region-wide flood events in January 1997. The plant community is slowly returning to the newly established floodplain and shade characteristics in those segments are indicative of that recovery. It is anticipated that recovery will continue and that shade levels and solar load levels will meet target reductions in those areas in the future.

DEQ has completed a TMDL for temperature in Big Willow Creek from its headwaters to the Payette River. In addition, DEQ is proposing to place Big Willow Creek on the state's list of impaired water bodies for flow and habitat alteration and to retain it on the list for unknown pollutants.

A public comment period on the draft assessment and TMDL document posted below closed May 4, 2007.