The previous blog left off with the Mt Rose SNOTEL maximum snow water equivalent (SWE) measured at the end of March and now headed downwards for April. The sad news was to enjoy the last bit of winter, as the days for skiing, snowshoeing, or sledding are limited. The good news? The river’s up!
If you spend a lot of time on the Truckee River, you may have noticed that the flow is increasing. The chart below shows streamflow as blue and yellow lines measured in cubic feet per second (cfs) starting in mid-January until today. The US Geological Survey (USGS) measures streamflow at gauges all over the nation and posts the measurements to their National Water Dashboard. Site 10348000 is along the “biggest little” Truckee River in downtown Reno.
The chart below is downloaded from the dashboard site for the downtown Reno gauge. Notice a flat blue line of about 500 cfs through most of January and February, followed by increases in the yellow line beginning in March, and doubling to the black dot of about 600 cfs today. It will keep increasing and maybe even double again in the coming weeks. The flow peaks around mid-May and then decreases into summer.
Look again at the chart and notice what appears to be a sine wave of streamflow starting at the beginning of April. The peaks and troughs show diurnal variation. Warmer temperature during the day increases melting while freezing temperatures at night decreases and stops melting. The temperatures we feel do not exactly line up with the peaks and troughs, as there’s a lag time between melting in the mountains, that melt entering the river, and then traveling all the way to the gauge at Reno. But you get the picture.
Previous blogs mention that snowpack measurements this year are among the lowest recorded. The means less streamflow, but how much less? It’s hard to say, but here is a frame of reference. Look at the chart again and notice a dotted black line above the yellow line – this is the median of all measurements made. A median is not an average; rank all measurements from smallest to largest and the halfway point is the median. A median is often used when you have a few really large values in a dataset, for example flood events.
Why not include flood events when looking at changes in river flow over time? Were you here in early January 1997? Check out the next blog for an explanation.