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Commentary: Where are Aliso Creek's missing miles?

February 14, 2013|By Elisabeth M. Brown

Seen from the air, the Mississippi snakes across the terrain in thousands of curves and loops. The landscape is dominated by circular and semicircular arcs, the remains of old meanders. Cutoff loops become ponds or lakes, called bayous.

A river's length and shape are determined by a number of factors: topography and volume of water are the most important. In its mountainous headwaters, a river is fast-moving; rushing straight down slope, rough with rapids, and carrying large boulders. Approaching the valley floor, the water slows in the more gradual terrain. As it loses energy, it drops the biggest rocks first, then ever-smaller ones. Eventually, it flows sluggishly, carrying only fine silt.

In nearly flat country, a river meanders through the landscape in large twists and turns. Meanders greatly widen the flood plain, the land subject to periodic flooding. This is favored farmland, fertile with the nutrients of deposited silt and easy to till.

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When land near a river urbanizes, people want to control the river to prevent further flooding. The Army Corps of Engineers builds and maintains miles of levees along the banks of the Mississippi.

In Orange County, with much smaller rivers and creeks, "flood control" means lining riverbanks and channel bottoms with concrete, straightening the channels and narrowing them. This deepens the mass of rushing water, and increases its speed and scouring power. More than half the length of Aliso Creek is subject to these man-made controls.

Aliso Creek drains a watershed of 30-plus square miles. Today it is 19 miles long, from the Santa Ana Mountains to the Pacific Ocean; originally, it was more like 21 miles. Hydrologists often say that if you shorten a river by straightening, it will try to get its missing miles back downstream.

The last few miles of the creek run through Aliso and Wood Canyons Wilderness Park, where the effects of channeling and increased water flow from urban runoff play out dramatically.

Through the park, Aliso Creek is down cutting, and can be 15 feet below its banks. Where we should see a vigorous streamside community of willows and mulefat, there are often only steep eroding earthen banks. A small dam was built in the 1980s to raise the water level and irrigate planted willows, but the planting failed.

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