Public Lab Research note

Erosion of civilisation, storm by storm

by neilh | April 14, 2012 01:48 14 Apr 01:48 | #1671 | #1671

Water quality is often degraded by simple erosion - resulting in excess sedimentation in creeks. The picture shows a little urban creek in Cotati Ca that I've been monitoring for 5years. The visibly brown muddy water coming into the creek is from Cotati Creek joining the slightly larger Laguna da Santa Rosa. The source of the erosion is about a mile away up a steep road - named "Water Rd". The ditches along "Water Rd" appear to have two management regimes - possibly two different municipal management styles one by the city (good) and the other by the county (problem). See "Two Different ditches". The stormwater ditch in the background is in the City i and has vegetation holding the soil together. The portion in the foreground seems to have had the vegetation removed (possibly chemical spraying) and the soil is eroding. The picture on "Water Rd Erosion" (county) shows how the erosion is happening - steep uncovered ditch sides with water running off, and then a close up of the results of the erosion. There are possibly 50 other pictures that could be taken like this. A complaint to the local water quality board two years ago resulted in some work and improvements by the county and a local land owner, but clearly its not enough. The last two pictures show the context of the first picture - why the water is running off - there has been about four storms over three days - all with increasing sharp amounts of rainfall. The hydrograph shows the runoff in the stream. The precipitation data comes from a low cost wireless weather station in my garden. It transmits readings to an indoor terminal, with a USB link to a win client EasyWeather ( that I download every 3 months and then can generate the precip graph in excel. It nicely shows that there have been a number of storms and as there is still sedimentation in the runoff its serious and persistent. The “serious” side is because its visibly degrading the water quality by more than 15% (a legal trigger), and the persistent is as a result of it still running off even after multiple storms. A day after the storms, the water clarity cleans up dramatically. Other pics at

Background reading for a favourite book - "Dirt: The Erosion of Civilizations" by David R. Montgomery


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