Give river room to roam: Development along river channel subject to flooding
Editor’s note: The following is the first in a series of articles by John Cobourn and Steve Lewis of the University of Nevada Cooperative Extension that deal with the Carson River. The articles will deal with flooding and water-quality issues, among other topics.
By John Cobourn and Steve Lewis
Special to the Nevada Appeal
One of the remarkably beautiful features of the Carson Valley is the river flowing through it. Viewed from Kingsbury Grade or Foothill Road, the Carson River and the ranches along its banks create a broad expanse of green vegetation that is especially lush in spring and summer.
However, as beautiful and important as the river is to the Carson Valley, its channel is unstable and likely to suddenly shift during major floods. Because of this instability, river experts warn people not to build structures close to the river’s banks.
Many property owners along the river are managing their land near the channel in ways that will enhance its beauty and reduce flood risks for future generations. Ranching and farming are excellent uses for riverfront property. When large floods, such as the 1997 New Year’s Day flood, hit the valley, open ranch land can store the water that rises over the riverbanks, slowing its flow and reducing potential flooding in communities downstream.
By using land near the channel for agricultural purposes, rather than building houses on it, the property owners are helping to prevent risk to lives and property in the future.
How the river channel has changed over time
One important reason that the land near our river channel is so dangerous for urban development is that the channel changes form during each large flood.
The channel has changed a great deal since the first European-American settlers arrived in 1851. At that time, the river meandered through a broad riparian, or streamside, swath of wetlands fringed by a gallery forest of cottonwood trees. During even small flood events, the water would rise out of the channel and spread across the valley floor, dissipating its destructive energy and harmlessly depositing sediment.
Over the past 150 years, human activities in the watershed have included logging, mining, ranching, road and bridge construction, straightening of the natural channel, building levees of various kinds and urbanization.
The cumulative effect of all the river alterations of the past 150 years is complex, but in general, the river channel has downcut, or incised, several feet into the valley floor. This has drained the former wetlands near the channel and lowered the surrounding water table. It has also made the banks so dry and unstable that the riparian wetlands and streamside forests have diminished or disappeared in many areas.
Why the Carson River is dangerous for development
In most places, the incised channel through the Carson Valley is a steep- walled gully with little riparian vegetation along many of its banks. During floods, water in the deep channel flows swiftly and creates enormous erosive forces on the riverbanks. These forces often collapse the banks, making the river gully wider with each flood.
Geologists who specialize in river-channel processes have found that property near incised channels is extremely unsafe for development. The theory of “channel evolution” holds that incised channels will continue to widen their gullies until they are wide enough to form a meandering channel and floodplain on the new lower level. A channel gully that is 10 feet deep and 150 feet wide today, such as the channel just east of Cradlebaugh Bridge on Highway 395, could be 500 feet wide in 10 or 20 years.
In the Carson Valley, much of the land near the channel is destined to be eroded away during floods. In some cases, the channel banks will move sideways tens or hundreds of feet during a single flood.
In his book “California Rivers and Streams,” Jeff Mount, chairman of the Geology Department at the University of California, Davis, says, “During very large floods, a river will occasionally abandon its meander belt entirely and establish a new channel in the surrounding lower-lying areas of the valley.” This kind of channel abandonment and movement is called an “avulsion.”
Property owners can keep development out of harm’s way
Because land near the river channel is so dangerous to build on, the Carson River Coalition, a group of concerned local citizens, is trying to create community awareness about the benefits of keeping residential development out of the river’s corridor. The corridor is the area that the river needs for meandering and widening.
Arnold Settlemeyer, a local property owner whose family has ranched here for generations, says that he and his family have witnessed the river changing location more than once in the past hundred years. He is concerned that the many new property owners who are building homes and roads near the channel may eventually find themselves in harm’s way.
• John Cobourn is a water-resource specialist with University of Nevada Cooperative Extension. Steve Lewis is the Douglas County Extension Educator with University of Nevada Cooperative Extension.