Paired-watershed Study

A paired-watershed study is conducted by using two similar watersheds and evaluating them for differences. One watershed is a control and the other has a management change applied to it. You can then measure the differences in flow and water quality exiting the watersheds, and have a pretty good idea of how the management change influenced any flow or water quality changes. Or in this case, with no management applied, the differences reveal natural differences in the watershed.

February 2020 was the driest February since 1864 in San Francisco. No rain fell there, as well as here in San Geronimo Valley (except for one weather station (WDAC1) that picked up 0.01 inches of precipitation on February 24th). December precipitation was 200% of average, however January was only 37% of average. February was 0% of average.

San Geronimo Creek peaked at a respectable 210 cfs in December--the highest December flow in 3 years, but less than 1/10th the peak flow in 2016--a good situation for Coho salmon redds, which got plenty of water but wouldn't have been washed out by flows high enough to scour the streambed. Since there weren't that many redds this year, a higher survival rate would be nice. January's peak flow was 70 cfs. February's was 6 cfs--perhaps the lowest February flow since 1864.

The larger watershed is to the left of this ridge, the small one is adjacent to that one.

The small dry stream on the 18th.

On the morning of February 18, I checked on two small steep upland intermittent streams a short distance apart that made an interesting paired-watershed study. One had surface water and one did not. The previous day's high temperature was 68 degrees F, and the morning low temperature was 35 F. On February 29th, I checked on them again. The previous day's high temperature was 71 F, and the morning low was 39 F. At the end of February, both streams had no surface water.

The larger wet stream on the 18th.

February 18
The bigger stream had water in it but almost no flow, and the smaller stream had no water in it--just moist soil. The differences between having water and having no water in a stream are stark. I was on a well-used deer trail that crossed both streams. Deer and other animals would obviously be able to drink at one but not the other. The larger stream had riparian vegetation and trees along it, yet the smaller stream had only coyotebush (arguably a riparian plant in this setting since they lined the stream). But clearly the larger stream was able to support trees when the smaller stream couldn't. This difference was due to natural characteristics of the two watersheds--primarily size, and perhaps soils and vegetation. But management changes can also cause a stream to go dry that would otherwise be wet.

The small dry stream on the 29th.

February 29
The bigger stream no longer contained surface water. The smaller stream looked the same as before. The lack of surface water in the bigger stream in February could be an unusual situation--but is it something that happens every 150 years, or is it more common? Only additional observations will tell. The implications of this for animals seeking water are significant. The stream network that provides surface water is contracting, and animals must now go downstream (or perhaps upstream) to find it. This concentrating of animals into a smaller area has implications for migration patterns, intensity of grazing, predator presence, and human disturbance--and in areas with roads and traffic, perhaps even roadkill patterns.

The larger (now dry) stream on the 29th.

The legal implications are also interesting to note. With federal protection for Waters of the U.S. narrowed under Trump, small intermittent tributaries like these could no longer be protected by the federal Clean Water Act (even though a federally-endangered species is present downstream and any degradation would be a cumulative impact). But if the drying of a stream is something that only happens every 150 years--or if it is happening for the first time due to climate change--does that stream lose its federal protections as well?

February 28, 2021 Update

2021 had another dry February, with only 2.07 inches of precipitation, and an overall drier water year than last year, at 13.63 inches (40% of average) of rain. At the end of February 2021, both creeks once again are dry, with intermittent pools in deep shady places--just like last year. Downstream, where they merge, there is ponded surface seepage through thick coast live oak leaves--no scouring flow occurred this year with every month having below average precipitation. Then, at the confluence with the next tributary, there is an audible trickle from both at the merge. Farther downstream, San Geronimo Creek had a peak flow so far this season of 140 cfs on January 27th.