A Far Too Rare Scene in These Parts in Recent Years: Flooding on Dry Creek

The latest storm to pass through Central California dropped 1.28 inches of rain in my backyard gauge, giving us 4.26 inches for the month of March, and about a third of a normal year's precipitation in the space of just ten days or so. By the time this last storm arrived, the watershed was saturated, and so Dry Creek...was not dry. It reached a peak flow of about 3,000 cubic feet per second yesterday.

 Unlike the Tuolumne River, there are no reservoirs like Don Pedro Dam to catch the runoff from the storms, so Dry Creek rises and falls quickly with the storms. The runoff shows two peaks, as there were a pair of storms over the weekend and Monday. The blue bars on the top show the amounts and timing of precipitation in the watershed, and the dotted line records the runoff. Notice the lag between the timing of the storms. This is the delay between the water falling from the sky, and the peak flow miles downstream. By analyzing the pattern and amounts of precipitation, predictions can be made as to the amount of runoff (the pink dotted line). If the storm is immense, graphs like this can be used to warn downstream residents to prepare for flooding, in terms of both the timing, and the discharge (water flow/height) of the river.

The rainy season is drawing to a close, although a few more storms will no doubt blow through. The drought is not over, but it is nice that all the precipitation arriving for the rest of the year is adding to the above-normal yearly rainfall total. If it could only be true of the snowpack too!

What Does An Urban Small Stream Flood Advisory Flood Look Like?

It looks kind of like this....

Dry Creek has its headwaters in the foothills of the Sierra Nevada north of Modesto Reservoir. It flows southwest through the farmlands into the town of Modesto, where it joins the Tuolumne River. Dry Creek, despite its name, is rarely dry. Prior to agricultural development in the last century it was indeed dry for much of the year, but since the irrigation began, seepage keeps water in the creek for most of the year. But rarely is there more than a few cubic feet per second.

Dry Creek is an unregulated drainage, meaning there are no dams or other measures to control floodwaters. It normally isn't a problem, but every couple of years events conspire to cause problems. This week was one of those weeks. The ground is saturated from weeks of above-normal precipitation, and we had a violent wet storm yesterday that dropped several inches of rain and hail in the foothills.

The creek won't stay high for long, but for a few hours today, the flow was over 3,000 cubic feet per second. By comparison, the usually much larger Tuolumne River (with the headwaters many miles away at the Sierra Nevada crest) is at flood stage at 9,000 cubic feet per second. With this discharge, some properties in downtown Modesto may be flooded. Pretty big numbers for a little podunk gully. I would have shown "before/after" pictures, but I never think of taking pictures of the creek when there is no flooding going on.

Stream data comes from NOAA, found here.