Tuolumne River to rise to make room in Don Pedro for storm runoff

The lower Tuolumne River ran at its highest volume past Modesto in half a decade Wednesday, and it will get even bigger over the next week.

Managers of Don Pedro Reservoir, 38 miles upstream, have sharply increased releases so the lake will have enough room for runoff from a weekend storm that could be huge.

As of 2 p.m. Wednesday, the Tuolumne flow had tripled to about 3,000 cubic feet per second at the Ninth Street Bridge, according to the California Department of Water Resources. It will reach about 7,000 cfs at the peak of the planned releases.

The river will rise high on the banks but not top them, said Calvin Curtin, spokesman for the Turlock Irrigation District. It operates Don Pedro in a partnership with the Modesto Irrigation District.

The Don Pedro releases are taking place under an agreement with the U.S. Army Corps of Engineers, which tries to ensure that reservoirs around the state have enough room for unusual runoff. It requires that Don Pedro’s surface rise no higher than 801.9 feet above sea level through April 27 of each year. It is at 830 feet when full.

This week’s releases come amid a forecast for heavy rain in some of the upper elevations of the Sierra Nevada. It could melt much of the lower-elevation snow that has fallen in recent weeks. An especially bad version of this happened in 1997, leading to major flooding in parts of Modesto and other locales.

The Stanislaus River does not have the same rise in volume this week because New Melones Reservoir, its main storage, is at only a quarter of capacity. McClure Reservoir on the Merced River was at 44 percent as of Wednesday.

“We are not releasing any water at this time,” said Mike Jensen, spokesman for the Merced Irrigation District. “We are watching the upcoming storm event and will be responding as appropriate.”

The lower Tuolumne last did this in spring 2011, shortly before the start of a drought that has not ended. At the worst of it in 2015, Don Pedro managers scrambled for whatever runoff they could store and farmers got only about 40 percent of their accustomed amount. The situation improved in 2016, but the current snowpack is still below average.

This week’s releases could have one benefit – clearing out the water hyacinth that has clogged parts of the river during the drought. Small mats of the non-native plant floated downstream Wednesday afternoon, as did a few plastic bags and other trash.

This story was originally published January 4, 2017 at 5:23 PM with the headline "Tuolumne River to rise to make room in Don Pedro for storm runoff."