Commonly Asked Questions

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.

Does water conservation benefit the environment?

A survey commissioned by TRT in 2018 confirmed that people conserve water with the expectation their actions will benefit the environment. But does it?

It can, but only if regulations are in place that require dam operators to release enough water to protect the environment. Currently, required flows on the Tuolumne are governed by baseflows – a minimum amount of water that must be left in the river. These baseflows are woefully inadequate, and can lead to lethal water temperatures that kill fish. Water agencies often release only enough water to meet their legal obligation, unless they’re fairly certain their reservoirs will fill by the end of the wet season.

As shown by the graph below, between 2012 and 2016 (five years), the Modesto and Turlock Irrigation Districts and SFPUC released an average of only 12% of the Tuolumne’s unimpaired flow between February and June, which are critical months for baby fish. Then, in 2017, the Tuolumne’s reservoirs filled quickly, and the water agencies had to “dump” the maximum amount of water allowed by flood protection rules from early January into the summer. Unimpaired flow in 2017 was 79%. The Tuolumne experienced one excellent year at the expense of five terrible years.

 Source: The Bay Institute

The water we conserved during the recent drought was simply impounded behind dams, and the Tuolumne suffered terribly. Had the Bay Delta Plan been in effect, unimpaired flow would have been 40% in each year between 2012 and 2016, and all the reservoirs still would have filled in 2017. You can see that the pattern has repeated throughout the years. 

To guarantee the water we conserver benefits the environment, the State Water Board must implement the Bay Delta Plan.

Did the SFPUC come close to running out of water during the recent drought?

No. At the height of the recent drought (2015), the SFPUC still had enough water in storage to last three years. In 2016 (an average water year) storage began to rebound, and by summer the SFPUC had enough water in storage to last five years. (Note: In an average year, the SFPUC is entitled to enough water to last three years.) 2017 was the second wettest year on record, and the SFPUC was entitled to enough water to last 12 years. Their reservoirs quickly filled to capacity, forcing the SFPUC to “dump” 88% of their entitlement.

During the drought, the SFPUC’s water bank in the Don Pedro Reservoir got low, but allowed them to keep their three main reservoirs close to full.