Conservation: Beaver Promote the Health of Coldwater Streams

by Ron Loehman, Conservation Chair

Rio GuadalupeOver the past several years I have been watching the progress of an example of natural riparian restoration on the lower Rio Guadalupe. The section in question is below the Gilman Tunnels and is characterized by a downcut stream channel with just a narrow band of willows along the banks that can provide shade to lower water temperature. Due to the downcutting, the area along the stream that was once flood plain now contains only upland plants such as Apache Plume, drought tolerant grasses, juniper, and Siberian elm.

In spite of the apparently poor habitat for them, there are beaver in this part of the Guadalupe. I’ve never seen one, but there is ample evidence of their presence in the chewed off stubs of coyote willow and occasional stashes of branches pushed into a muddy bank. Property owners further downstream report that beaver at times come all the way down to feed on their trees at night. Russian olives seem particularly attractive. Until the past couple of years, however, there has been no sign that the beaver have been applying their well-known hydraulic engineering skills to modify the stream.

beaver2Starting about three years ago I began to notice that one of the streamside trails that I hike was occasionally flooded. Since this was in an area separated from the stream by at least a hundred feet of really dense Apache Plume growth I didn’t immediately explore the cause and instead just started using a detour. Over time, the flooding has become more extensive and more permanent. The vegetation in the flooded area has changed significantly over time. Where before it was dry and brown it is now lush and green. Willows have sprouted up where none were growing before. There are hummocks of some of the tallest side oats gramma grass that I’ve ever seen. The area clearly is transforming to a marshland. The apparent cause is the beaver, but since the area is fairly inaccessible it was difficult to tell for certain.

beaver3About two months ago I decided to find out how this change was being accomplished. Because thick vegetation in the wet area made it nearly impenetrable, I put on my waders and entered the stream well below and made my way upstream. As I walked up the Guadalupe I began to see where small side channels dug through thick streamside vegetation were returning water to the river from some sort of upstream diversion. The returning water was clear and cold. Because of the river gradient and the downcut banks, it was hard to imagine how beaver could get the water four to five feet uphill to the marshy area, so I began to suspect some sort of human intervention. However, I underestimated the beaver.

Eventually I rounded a bend and saw an amazing sight- a chest-high beaver dam. The beaver had accomplished the water diversion by raising the river level upstreamof their dam by about four feet. Above the dam they have dug outlets that run the water out of the stream and through a network of channels that flow down parallel to the river, creating the marsh. The water depth above the dam is only a little greater than below, so the dam is retaining about three feet of sediment. It seems unlikely that the dam was built in only a single season. I suspect the beaver have progressively raised the dam height over several years until the water level was high enough to begin their diversion project.

beaver4None of these observations about beaver is new and I’m sure that they will seem old hat to many of you. It has excited me because I have been able to see their work gradually unfold to produce amazing results in a relatively short time. The beaver will do what they have evolved to do if we just let them get about their business. So far I haven’t seen evidence of increased numbers of trout, but it may be too soon. That stretch of the Guadalupe doesn’t have many fish because of high summer water temperatures, low amounts of cover, and probably limited food sources. The beaver engineering should improve all of those characteristics.

In conclusion, here is a short summary of some of the ways beaver benefit cold water streams.

  • Beaver dams raise the water table and promote riparian vegetation growth.
  • Overhanging willows and other riparian vegetation provide shade and lower water temperatures.
  • Riparian vegetation supports insect life, which is food for fish.
  • Beaver dams retain sediment and reduce turbidity downstream.
  • Beaver dams and associated marshes retain, slow, and disperse seasonal runoff water, which promotes aquifer recharge and keeps streamside springs flowing longer..
  • Beaver ponds provide food and shelter for fish and other aquatic wildlife.

Support Your Local Beaver!

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