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Rain, snowmelt have filled Tuolumne River with debris

A debris boom captures logs washed by recent heavy rain into the Tuolumne River upstream from Don Pedro Reservoir.
A debris boom captures logs washed by recent heavy rain into the Tuolumne River upstream from Don Pedro Reservoir. Don Pedro Recreation Agency

Modesto and Turlock farmers are thankful that record storms have boosted to capacity Don Pedro Reservoir, which holds water needed for crops. But excessive rain and snowmelt also have washed huge amounts of debris into the Tuolumne River upstream from the reservoir.

Logs and branches covering from 3 to 5 acres typically end up in the river each spring. This year’s debris field is much worse – 30 to 35 acres, so far – because the extra runoff is bringing with it tons of gunk and trees killed in the giant Rim Fire of 2013, which ravaged about 40 percent of the Tuolumne’s mountain watershed.

“You don’t want that stuff making its way into the reservoir,” said Modesto Irrigation District board member Jake Wenger. While Don Pedro is an important source of recreation and farm water, some also is treated and sold to Modesto which mixes it with groundwater for the city’s drinking water.

The amount of debris this year is significantly higher, a lot more than usual.

Jake Wenger, MID board

Fortunately, logs float. Water-top booms gather them in big floating masses. A tug boat then shoots a cable from a ship-mounted cannon to the shore, where winches pull the debris in tight bunches to land.

When summer comes, the river level will drop, beaching the debris, which crews gather into piles for burning when they’re good and dry, in the fall.

The reservoir – and the river’s debris – are managed by the Don Pedro Recreation Agency, a partnership of the Modesto and Turlock irrigation districts, and San Francisco, which also depends on the Tuolumne for tap water.

MID and TID agreed after the Rim Fire to see how ash and soot might affect the river and reservoir, and hired the U.S. Geological Survey to find out. Three years of monitoring yielded “no substantial impacts to water quality due to the Rim Fire,” an MID report says, but the monitoring stopped before big storms hit earlier this year. The partners will talk about continuing in coming months.

Garth Stapley: 209-578-2390

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