What is unimpaired flow?

Unimpaired flow is the amount of water that would flow down a river in the absence of dams and diversions. Unimpaired flow is important because it influences critical factors that affect fish and wildlife, including water temperature, inundation of floodplain habitat, water quality and fish migration.

In 2010, the State Water Board released a flow criteria report that determined the lower San Joaquin River and its three main tributary, of which the Tuolumne is the largest, would require 60% of unimpaired flow between February and June to protect fish and wildlife.

In an average year, unimpaired flow on the Tuolumne is only 21% between February and June. On December 12, 2018, the State Water Board adopted a range of unimpaired flow between 30% and 50% (starting at 40%) on the lower San Joaquin and it’s tributaries. While these requirements have been adopted, they have yet to be implemented, and a number of water agencies, including the Modesto and Turlock Irrigation Districts, the SFPUC and BAWSCA, have sued to block the plan.