New Water Development by Enhancing and Restoring Beaver Dam Complexes

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by Butch Clark


New water development should solve more problems than it creates. New water development should link water quantity with water quality. Hopefully, new water development should make both environmental sense and economic sense. Restoring and enhancing degraded stream channels by restoring old beaver dam complexes and wetlands improves water quality. A challenge is to do this and acquire water rights.

The ability to obtain a water right for restoring or enhancing a beaver dam and wetland complex provides valued “legitimacy” to the effort. The difference between costs for restoration and costs for constructing the equivalent amount of traditional water storage can fund restoration projects. Finally, with a water right, there is likely to be greater commitment toward maintaining the restored conditions and the project.

Virtually every headwater stream in the West had some beaver dams before trapping and settlement activity removed the beavers. However, the beavers left behind a legacy. This was the shallow aquifer composed of sediment trapped behind the beaver dams along small stream channels — those of the 1st, 2nd, 3rd, and even 4th orders. These streams tend to be flashy with high flows during snowmelt or summer storms. At other times the flow is just a trickle or none at all. Water stored and released from the aquifers sustained water flows and riparian vegetation.

Now, in many of these places, streams are downcutting gullies or arroyos. Riparian vegetation was lost as incised stream channels lowered water tables, and such stream situations are now characterized as degraded. Beavers maintained the former wetlands and riparian areas; removing the beavers removed the maintenance crew. Overgrazing and other settlement activities contributed to downcutting of channels and accelerated erosion. Now these streams present non-point source pollution problems. The need to control erosion from streambanks and adjacent lands is evident. Sediment flushed down the gullies creates many problems for water users and wildlife below. Almost no water storage occurs to sustain late season flows or even trickles.

Studies following reintroduction of beavers to such degraded sites note appreciably greater and longer summer flows and elevated groundwater levels after the beavers’ return. Some ephemeral streams have started to flow throughout the year. Much the same is noted in studies of small erosion control structures such as check dams and sediment traps. Water was being stored in the aquifer during times of high flow and slowly released later.

Literature on this topic is growing rapidly. Most notable are reports since the mid-1980s from the University of Wyoming’s Water Resource Research Center. These offer a particularly rich source of ideas and very practical advice. However, in the 1930’s the US Department of Agriculture advocated methods to recharge aquifers artificially for underground storage.

Water can infiltrate streambanks or percolate downward through the soil to the aquifer faster than it travels horizontally through the aquifer and returns to the stream. Small structures — those made by beavers, humans, or both in partnership — reduce flow velocity, trap and stabilize sediment, and cause water to infiltrate streambanks for storage. The dams built by humans are about a foot and a half high. Required materials are old tires, woven wire, erosion mat, and steel posts. Success has been about 95% for many hundreds of small dams. Reports from the University of Wyoming give detailed instructions and advice. These and other reports also provide needed information about how to measure and monitor the changes in ground water storage levels, usually with numerous small diameter plastic-pipe wells.

Efforts by beavers are a little more sophisticated and their list of materials is different. Also, their instructions are harder to translate. However, both groups of builders find that structures work best when plant roots are encouraged to bind together the construction materials. Beavers generally are willing partners on projects undertaken by humans, given the opportunity.

When trapped sediment fills in behind a structure, a new one is built just upstream in “stair-steps.” Gradually accumulated sediment behind the dams will raise the level of the streambed. Eventually high water flows can overtop the channel banks and begin spreading across the long-dry floodplain. The ground water table rises, and riparian vegetation reappears. The process of infiltration/percolation, storage, and lagged return is what brings back improved or perennial base flows to degraded streams following restoration.

How much water can be stored in the aquifer will depend on site characteristics. How fast this stored water returns to the stream channel depends on aquifer characteristics and how far this water is made to travel horizontally through the aquifer before it is allowed to reappear and join the surface flow. Riparian vegetation shades the soil, lowers wind currents, and reduces soil compaction. In combination this increases percolation of snowmelt, rainfall, and flood water through the soil and into the aquifer beneath for storage. Reports suggest recharge of the aquifer exceeds the water loss by increased evapo-transpiration from riparian vegetation.

Capital cost in 1988 dollars for human structures in Wyoming was about $10.85 per acre-foot of created storage. This contrasts with recent estimates of about $8,000 per acre-foot for constructing traditional reservoir storage facilities. In addition, restored wetlands and riparian areas also improve water quality. The natural services they offer include filtration, adsorption, volatilization, precipitation of dissolved solids, microbial decomposition, and vegetative uptakes of pollutants. They attenuate flood flows from storm events and snowmelt. The water they capture and store is later released gradually. They also trap sediment. An economic valuation of these beneficial and natural services totaled $2,000 per acre per year in 1972 dollars.

To acquire a storage water right, water must be diverted or removed from the normal course of flow. The “normal” course of flow is down the incised stream channel. Beaver dams and structures installed for restoration divert water from the normal channel to the underground aquifer storage. Water infiltrates through the streambanks and percolates down through the soil as it flows across the flood plain. Once water is present in the aquifer, it is possible to estimate the quantity of storage and the timing of the water return to surface flows. Both can be designed into the restoration project to serve intended beneficial uses such as late season irrigation or uses associated with augmentation plans.

For example, a degraded stream channel can have an elevation drop of 200 feet over a mile. On either side and beneath it is a sandy-gravelly sloped aquifer of trapped sediment about 30 feet thick. If the aquifer extends over about 98 surface acres along the reach, it has the capacity to actively store and release about 580 acre-feet of water. Design for restoration must match the travel time of water stored in the aquifer with the requirements of water development for the intended beneficial water uses and users. Careful measurement and monitoring is essential.

Humans and beavers can work in partnership to restore what beavers left as their legacy. The partnership can improve water quality, improve wetland and riparian areas, and should receive water rights for storage– and all this at less cost than for most traditional development projects. Sometimes old ways are BEST (Beaver Embodied Storage Technologies). Useful References:
Cairns J. Jr., as Chair (1992) Restoration of Aquatic Ecosystems: Science, Technology, and Public Policy, report of Committee on Restoration of Aquatic Ecosystems of the National Academy of Sciences, National Academy Press, Washington, D.C., 552 pages.

Linsley J. S. and Franzini J. B. (1979) Water Resources Engineering, 3rd ed. international, McGraw-Hill Book Co., Singapore, 718 pages.

Muckel D. C. and Schiff L. (1955) Replenishing Ground Water by Spreading in The Yearbook of Agriculture – 1955, pages 302 – 310, US Department of Agriculture, United States Government Printing Office, Washington, D.C., 751 pages.

Skinner Q. D., Smith M. A., Wesche T. A., Dodd J. L., and Rodgers J. D. (1991) Research Findings from Riparian Zone Management on Muddy Creek in Proceedings of the Third Intermountain Meadow Symposium (E. G. Siemer, Ed. ) pp. 47 – 56, University of Wyoming and Colorado State University, 1991, Fort Collins, Colorado, 254 pages.

Stabler D. F. (1985) Increasing Summer Flows in Small Streams Through Management of Riparian Areas and Adjacent Vegetation: A Synthesis in Riparian Ecosystems And Their Management: Reconciling Conflicting Uses – First North American Riparian Conference, April 16 – 18, 1985, Tucson, Arizona, GTR RM-120, pages 206 – 210 (R. R. Johnson, C. D. Patton, and R. H. Hamre, as Tech. Coords.), USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, 1985, Fort Collins, Colorado, 523 pages.