As residents throughout the Midwest are grappling with the effects of massive flooding from the Mississippi, questions are being raised about how the biggest river in the U.S. has changed throughout history. Man-made efforts to stem the tide of the mighty Mississippi have made the river run deeper, faster and narrower; and the number of floods has drastically increased in the last 50 years.
Right here in our Ross Valley, on a much smaller scale, a very similar history is playing out.
Laurel Collins, a geomorphologist, has given a fascinating and information presentation on the history of the Corte Madera and San Anselmo Creek at Flood District 9 meetings twice in April. The video of that talk is now available on the flood district's website under "History of Corte Madera Creek."
The San Anselmo Creek, says Collins, basing her research on historic newspaper accounts and photos, had a very different course before 1867, which can be seen in the pictures at the right. A massive storm created a blockage near Center Boulevard and Madrone Avenue, redirecting the creek through a number of smaller distributaries.
Hard, older rock in the topographical makeup of the soil coming out Sleepy Hollow dictated where the creek was redirected.
At the same time, people were pouring in to San Anselmo, eager to build houses and stores. They had little knowledge of the historical flood plain they were building on. The small, new creek arms seemed unthreatening and buildings throughout downtown were erected directly over this tiny creek. But, as the creek readjusted to its new course, those buildings how find themselves continuously threatened by flood.
The train line, built out in the late 1870s, further complicated matters. As anyone who drives down Center Boulevard could tell you, the train line (and now that street) were built about four-feet higher than the ground. This prevented the new train lines from flooding, but it also trapped the redirected creek in between hard rock and a dam-like berm.
Trapped, the creek -- which had once been shallow with low-level flooding on a much wider plain -- became deeper, faster and unable to spill out over its banks in a controlled manner. The low-level flooding that occurred regularly throughout the Ross Valley had historically shaped the valley and distributed dredge from the creek throughout the area. But, the creek could no longer top its banks, so the water ran faster and eroded under the buildings that had been erected over its banks.
Since 1867, according to Collins, there have been 16 to 18 large floods (depending on what you count). Just since the early 1980s, San Anselmo has experienced two 100-year floods. Though hard to document, it appears devastating floods of this nature were rarer before 1867.
Watch the video to learn more about how the train companies built a ditch to redirect the creek through Ross and the changes that occurred near Woodlands Market.
