Wednesday, August 17, 2011

A Lesson in Hydrology

This project on the Pawtuxet River has been a good lesson in watershed hydrology. As the work crew was at the site on Monday, preparing water control for removal of the last section of dam, rain was falling steadily and water levels rose as they were working. It became clear by the middle of the day that there would be no more work in the river until water levels fell.

Looking at this hydrograph from the USGS gage in Cranston, you can see the flashy nature of this river system and how it responds to rain events. The first rain event we had on Sunday, August 7th, rose water levels to over 800 cfs. Flows were also being manipulated by a hydro operation upstream. The several inches of rain we had on Monday were enough to increase flow from under 200 cfs to over 1,000 cfs within a few hours. Because the watershed is so large, flows have still not dropped substantially two days later. Some of this is due to the fact that the Scituate reservoir is now full and is spilling over its dam. Flows on Monday were the highest for that day since August 15th 1973 when a flow of 827 cfs was recorded at the Cranston gage.

Rivers respond to what is happening in the watershed. A large amount of impervious surface causes quick rises in river flow. Manipulation from dams and other human influences also cause flow fluctuations putting added stress on the river's ecology and built environment. When flows reach around 500 cfs, we will see the crew back at work on the last section of dam.

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