The missing miles of Aliso Creek

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, fossils of past meanders. The Louisiana bayous are old loops now cut off from the main river channel to become lakes and wetlands.

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, the river is fast-moving, rushing straight down slope, rough with rapids and carrying large boulders. Approaching the valley floor, it loses energy and slows, dropping the biggest rocks first, then 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, and the land is subject to periodic flooding. This is favored farmland, fertile with the nutrients of deposited silt and easy to till.

When land near a river is farmed or developed, people seek to control the river to prevent further flooding. Levees are raised along the banks of the Mississippi. In Orange County, we usually line the riverbanks and channel bottom with concrete, straightening the channel and narrowing it. 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-something square miles. Today it is 19 miles long, from the Santa Ana Mountains to the Pacific Ocean; originally, it was closer to 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 Woods Canyon Wilderness Park, where the effects of increased water flow and upstream shortening play out dramatically.

Through the park, Aliso Creek is down-cutting rapidly, as much as 15 to 20 feet below the adjacent banks. Where we should see a vigorous streamside community of willows and mulefat shrubs, there are only steep eroding banks. A small dam was built in the 1980s to raise the water level and irrigate planted willows, but the project failed.

The creek strains at the edges of the channel, trying to lose excess energy by meandering, and further erodes the banks. This exposes the underground pipes of the sewage treatment plant, forcing emergency concretizing.

By the time it reaches Ben Brown’s golf course, the frustrated creek spills over its banks, given half a chance. I recall several times when mud covered the golf course for weeks or months. When the eastern toll road was built, the creek carried bright white clay from the site where the road crosses El Toro Road and deposited it on the golf course.

A river is a dynamic system with a will dictated by the physical laws of water flow. Engineers talk about “taming” rivers with dams and levees, but ultimately they can’t be changed, only accommodated.

In the name of expediency, the new owners of the golf course now plan to raise all structures above the flood plain. To give the creek back its missing two miles would be a lot harder.20051118h9vgwqkf(LA)