Size Of Main Vernal Pool and Human-Caused Changes With Time

See Vernal pools of the Santa Rosa Plateau for an introduction to the Main Pool of the Santa Rosa Plateau.

The largest Mesa de Colorado pool is roughly 25 acres, as determined from GPS measurements in February 2002, and is one of the largest in the state. This size agrees with the detailed measurements made by Lathrop and Throne in 1983.

There is considerable variation in the reported size of that pool, probably because the size of the pool has changed with time. Its size has been reported as ranging from 10 acres (California Department of Fish and Game) to either 25 acres (from San Diego Union-Tribune article, May 9, 1996) or over 30 acres (from California Wild Lands).

The pre-human size of the pool was about 12 acres. When water is absent, the original pool still shows up prominently on aerial maps, and as a result is in fact what is mapped on the USGS Topographic Map. Sometime after 1909, when the Vails took title to the Santa Rosa Plateau and began running cattle on it, they dammed the stream outlet of the pool. Sandy Wilkerson a Vail grandson, reports the dam was built in the 1920s.

When the dam was built, the upper part of the stream outlet was converted into part of the pool. Over the ~75 years that the dam existed, the stream outlet gradually filled with sediment, with the sedimentation rate undoubtedly increased by cows trampling through the area.

Even though the dam was breached on 2 December 1990, the pool has remained at the larger size due to the sedimentation. Interestingly, that part of the pool had the densest concentration of non-native invasive curly dock, Rumex crispus, prior to its removal in 2006 and 2007, confirming that the properties of the soil are different there.

The following aerial view, from Google maps, shows the changes:

As you can tell from the above, this pool is a severely disturbed pool, having undergone significant changes to its drainage regime in the last hundred years.

In 1995, a permanent boardwalk and bench, made out of recycled plastic, was installed at the largest Mesa de Colorado pool, allowing visitors to see the pool's activity close-up. At the center of the pool, the boardwalk encircles about a 6' diameter area of the pool.

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Copyright © 2008 by Tom Chester.
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Updated 14 January 2008.