spring snowmelt 2017

Spring Snowmelt and Flow on the Tuolumne River

Given the intense heat over the last few days, it is hard to remember that today is only the first day of summer.

Our wild and wet winter included some intense rainstorms but also left oodles of snow in the high country. The rate of snowmelt has increased as the weather heated up during the spring.

Daily flow data is measured or calculated at a number of stations, inlcuding those shown below. Water managers, rafters and kayakers, and yours truly keep close track of what is happening in the watershed.

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During late spring, most of the Tuolumne's flow originates in the high country above Cherry Reservoir and Hetch Hetchy.

The (data rich) chart below plots daily values for 

  • the flow of the Tuolumne River just above Hetch Hetchy, 
  • the flow of Cherry Creek the Tuolumne's largest tributary,
  • the total "unimpaired" flow of the Tuolumne (calculated at La Grange as if there were no dams), and  
  • the temperature at Tuolumne Meadows (8600 feet).

snowmelt_chart.pngSome of the early spikes in flow are due to rain, with decreased temperature (April 7 for example). As the spring progressed, however, Tuolumne River flows have corresponded largely with warming temperatures which melt snow in the high country.

The highest flow, 20,149 cubic feet per second, occurred on the last day of spring as the average temperature at Tuolumne Meadows reached 60 degrees (up to 80 degrees during the day but down to 42 degrees at night)

Note that in the late spring, the majority of the Tuolumne's flow comes into Hetch Hetchy along the main river and into Cherry Reservoir. By May, rainfall and lower elevation snowmelt is less of a factor. 

July flows are expected to be about one half of what we have seen in June. By August, the snow will be mostly gone and we can start thinking about what 2018 will bring.