The federal Clean Water Act requires that states and tribes restore and maintain the chemical, physical, and biological integrity of the nation's waters. States and tribes must adopt water quality standards necessary to protect fish, shellfish, and wildlife while providing for recreation in and on the waters whenever possible.

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 pollutant loads set at levels to achieve water quality standards.

Cascade Reservoir is located in the Payette River Basin of southwestern Idaho. Two rivers (North Fork Payette River and Gold Fork River) and four creeks (Mud Creek, Lake Fork Creek, Boulder Creek, and Willow Creek) all discharge into the northern end of the reservoir. The watershed includes two major drainages: the Big Payette Lake drainage area and the Cascade Reservoir drainage area (the area included in this management plan).

Cascade Reservoir is a shallow water body, and, as such, it is highly susceptible to eutrophication due to nutrient loading and elevated summer water temperatures. Eutrophication was especially apparent in 1993 and 1994 when dense mats of blue-green algae were present in the reservoir. Twenty-three cattle died in 1993 after ingesting toxins produced by the algae and a substantial fish kill occurred in 1994.

Cascade reservoir has been identified as water quality limited due to violations of water quality standards for dissolved oxygen, temperature, and pH. Elevated levels of phosphorus directly contribute to these violations. To improve the quality of water in Cascade Reservoir and its tributaries, the current contribution of phosphorus from external sources must be reduced by 37%, and this reduction must be maintained for at least five years.

The plan developed for achieving water quality improvements in Cascade Reservoir includes three phases and an implementation plan: