Friday August 25, 2023

South Yuba River Citizens League

Dams create a number of issues for the overall health of a watershed. One of the primary issues is that they modify the volume (discharge) and timing of when and how water flows in the portion of the river downstream. Dams trap and store water in the wet periods of time, and then release additional water during the dry, hot periods of time to meet human need.  

The graph which shows the amount of water flowing past a point over time is called a hydrograph. There are no specific guidelines for how much time a hydrograph has to represent. They can be for a single storm, a calendar year, a water year (October 1 through September 30), or for the entire period of record.  

The portion of a hydrograph where the discharge is increasing is called the rising limb, the highest point on the hydrograph is called the peak, and the portion of the hydrograph where discharge is dropping is called the falling limb. Depending on the period represented, this pattern of rising limb, peak, and falling limb can happen multiple times. In natural, snowmelt dominated systems, like the gage on the North Yuba at Goodyear Bar, we see a rising limb, peak, and falling limb every 24 hours. This is called a diurnal. The snow melts during the day and works its way into the river. The discharge peaks in the middle of the night, and the discharge drops back down. 

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