Natural Dam Site

Natural Dams

Beaver dams are an important natural phenomena. The lakes, ponds, wetlands, and meadows formed by beaver dams increases bio-diversity and improves overall environmental quality. The extent of these positive effects has been radically diminished greatly since beavers had been virtually eradicated from the United States in the early 1900’s. Modern society has recently begun to realize the benefits of wetlands. This realization marks a turning point in over 300 years of aggressive wetland drainage. Beaver dams are the primary natural method of establishing new wetlands. Beaver dams represent the only natural method of forming lakes, ponds, and wetlands in most watersheds. The exceptions to this would be glacial lakes, or lakes formed by geologic activity. North America will never be able to allow beavers to restore to their original range, but it is important to realize that changes to the landscape that are presently occurring in their absence.

Photograph of a small beaver dam about 5’ high, perching an estimated 50 acre feet of flow reserve on a trout stream tributary, near St. Paul Minnesota. Discharge stream below feeds into a trout stream.

The first effect of loss of a beaver dam is the drainage of wetland and the lowering of the water table above it. The second effect is the scouring and erosion that occurs in the channel that once was dammed. This scouring and channel deepening can be called a “ditching effect.” This ditching effect further lowers the water table and increases runoff rates. The lowered water table negatively affects vegetative cover. This process amplifies itself. Of course, this system will eventually come to a new equilibrium, governed by the ability of the landscape to withstand the erosive forces. The problem is that these effects have not yet stabilized, and that erosion will continue and runoff rates will continue to increase. This phenomena is everywhere in various stages, but can be best seen in meadow areas where erosive channels are currently migrating up the watershed. In some cases the meadows have already washed away. Numerous changes have occurred with the loss of these naturally occurring dams. Consider the many benefits of beaver dams.

Benefits of Natural Dams or Beaver Dams

Nullifies “ditching effect” on water tables caused by deepening river and stream channels.
Reduces channel scouring and stream bank erosion.
Erosion mitigation.
Reduction of sediment loading in streams and rivers.
Development of new wetlands.
Increased biodiversity including a better environment for fish and waterfowl.
A more stable water supply for wildlife, and vegetation.
Ground water recharge and ground water table elevation.
More cold water springs charging rivers and lakes.
Longer land water retention time in water cycle since subsurface flow is slower than stream and river flow.
Flood mitigation due to increased ground water holding capacity. (More capacity then the ponds themselves!)
Dampening of stream flow rate variations and stream charge during drought cycles.
Formation of natural lakes and ponds, and maintenance of existing ponds.
When dams ultimately silt in, natural fertile meadows form.
Stills and deepens waters, improves canoeing.

Beaver

Illustration credit: Reston Association

Modern agricultural drainage has had less effect of wetland reduction, than the original removal of the beavers. Agricultural drainage in the form of ditching and tiling is a relatively new phenomenon, so the cause and effect of changes can be better quantified. We know of no scientific articles that have actual hydrologic data describing the effects of removal of beaver dams on a large scale. A visualization experiment may be useful. What do you think removal of 250,000 water retention ponds and wetland areas per State in the Unites States would have on: 1) Flooding; 2) Groundwater recharge and quality; 3) Maintaining constancy of ground water tables and streams levels in periods of drought? Donald L. Hey has written an excellent scientific paper on this topic that was presented to the Annual Meeting of The American Institute of Hydrology 2001 titled, “Modern Drainage Design: the Pros, the Cons, and the Future.” Many modern watershed decisions have worsened flooding and drought effects. Our watershed management decisions must be made in the context of understanding the original extent of the effects of beaver dams.

One specific example of modern scientific misunderstanding of the positive effects of beavers is on the topic of stream bank erosion and stream meandering. Numerous textbooks state that stream meandering is caused by physical processes seeking equilibrium energy dissipation rates. It is also taught that equilibrium will be achieved when the rate of streambed erosion equals the rate of deposition. Given that beaver dams dissipate flow energy, and change channels into stilling pools, why aren’t there chapters on beaver dams in most geomorphology textbooks? Stream channels would be more stable as still interconnected ponds with energy dissipating steps. Topographical maps provide evidence these ponds existed before the beaver dams were removed. Currently, these areas are far from achieving a state of “equilibrium” and will continue to scour (deeper and wider). One alternative method to stop stream bank erosion and meandering would be to restore beaver dams in these erosive meandering areas.

Still water in beaver pond.

Still waters above the discharge stream in the previous photograph.

The following illustration depicts how beaver dams in stabilize stream flow rates. The illustration shows a horizontally compressed cross section between two streams, and how groundwater charge keeps the stream flowing. The river channels are the “U” shapes and the water flows towards you. Groundwater charge is the reason streams continue to flow without inputs such as rainfall. Water will continue to fill the stream until the level of the black triangles is reached. The top illustration shows the surface profile, and the groundwater levels for typical rainfall conditions with no beaver dam. The bottom illustration shows the elevation of the groundwater table under the same typical conditions with a beaver dam present. Beaver dams naturally leak, so the stream will continue to be fed until the level of the black arrows are reached. Notice that the “typical reserve” is greater in the bottom illustration, and that an additional storage buffer exists for wetter conditions. This wet condition buffer is represented by the white area “full dam reserve” and provides storage for flood mitigation. The blue area is the water charge, and the curved top is caused by rainfall. The effects of beaver dams in increasing the charge of aquifers reaches (sideways) across to the next watershed, and upstream as far as the pool is raised! The increased “typical reserve” behind a beaver dam is of significant benefit to wildlife and fish during periods of drought. The benefits are also seen downstream since beaver dams inherently leak as do charged aquifers. Cool springs are caused by water flowing out of charged aquifers. These springs can occur above and below the stream surface.

Beaver Dam Effects on Watershed Subsurface Water Reserve

Beaver Dams raise groundwater levels, and create storage buffer.

Cross section slice representing two streams (streams flow out of the page). Blue ground water reserve keeps streams flowing until the reserve reaches the stream level as indicated by the black arrows. Notice the increased ground water reserve with a beaver dam!

Photograph of mossy spring, water is coming from rocks.

Cold water spring in proximity to beaver dams. These springs also occur in streams but cannot be seen!

Beavers covered most of North America prior to 1900. It is estimated that over 60 to 200 million beavers populated the range shown on the map. The beaver’s influence touched every watershed in North America. Assuming 100 million beavers in the United States and 8 beavers per dam, there may have been an average of 250,000 ponds per state! Beaver dams significantly influenced erosion/deposition patterns over the entire country. The sediments that were dislodged from the naturally vegetative covered land were often recaptured in the natural stilling ponds created by beaver dams. Water after passing through beaver ponds and wetlands was of better quality with reduced sediment load. The natural energy dissipative characteristics of the spill side of these dams further reduced the erosive potential of flowing streams. Erosive energy was dissipated in the seepage through, and over the beaver dam.

North American Range Map

In the 1805 Lewis and Clark expedition up the Missouri River, beavers were observed wherever the habitat was suitable for them (EPA News-Notes). The water transportation systems of the Native Americans also must have been assisted by the numerous beaver dams. The elevated water tables also improved the vegetative ecosystem.

It should be clearly stated that beaver dams pose no unnatural hindrance to fish migration and are actually beneficial to such native cold water fish as trout. Beaver dams were the norm prior to 1700 in North America, fish and beavers had to have evolved together (reference above 250,000 dams per state)! The height of beaver dams is typically less than 10 feet. Fish migrations are seasonal, and typically occur in the springtime. In the spring high flows often overtop dams, and the downstream water level approaches that of the upstream side of the dam. The fish that evolved under pristine conditions in North America can easily swim over dams in these conditions. These flow conditions in the northern latitudes usually occurred in the spring when the water was colder. This presents a clear advantage to trout and similar native species over warm water species such as carp (non native). The temperature of the water charge during low flow periods will be cooler given the fact that low flows in rivers are the result of groundwater flow. In most climates low flows (droughts) occur during the summer season. Groundwater most always recharges rivers and streams during droughts at the average seasonal temperature. Trout seek these cool spring fed areas during the warmer weather.

These small trout all sought the deepest water in a Minnesota trout stream - Just above a beaver dam.

Trout in stream below dam in first picture

Geomorphology is the study of changes of the earth’s surface over time. A number of plants and animals have a significant effect on the type of changes that will occur. Prairie dogs, for example, create natural rainwater drains to more effectively recharge groundwater. Earthworms significantly affect the ability of the soil to absorb water during a rainfall event. Trees, grasses and other vegetation stabilize soil. Tall prairie grass in particular tends to enable the filling in of “micro gullies” that if unchecked would become larger gullies. This grass “lies down” during overland flow, protecting the soil, and allowing sediments to fill in small erosive starts. Beavers work on a macro scale creating ponds that support other life forms including fish and waterfowl. The natural sedimentation in beaver stilling ponds reduces downstream sedimentation, and ultimately forms flat fertile wetland and grassy areas called “Vegas”. The term Vega is Spanish for fertile valley, and refers specifically to a silted in dam or natural beaver meadow. UNM Sevilleta LTER Vegas occurred more commonly in mountain areas where erosion rates were naturally higher. Ranches, farms and cities were built on these natural flat fertile areas. Beavers had to be reintroduced in some of these Vega areas to stop the erosive processes that greatly accelerated after the beavers were removed.

The natural geomorphologic outcome for continents without beaver dams will include more ravines and steep valleys, due to the cutting erosive forces of flowing water. As inland river channels deepen, streams that flow into the main river will form. These streams concentrate the precipitation flow, which increases the scouring (deepening) of the river channel. This deepening effect amplifies itself. This is the reason that rivers form. The deeper channels increase erosion rates, leading to distinctive ravine topography. The ultimate result of this system will be low and flat topography, with the finer sediments washed into deltas. Beavers instinctively build dams in areas of more rapidly moving water, which reduces scouring – reducing channel deepening. Beaver dams typically bring the water surface to the top of the riverbank. The sediment deposition in beaver ponds also counteracts scouring (channel deepening). Prior to 1700 many streams and rivers may have been actually a series of ponds with steps (dams) between them. Early geologists observed this step topography. A very large number of beaver dams will shift precipitation flow from rivers and streams into more overland flow, and underground flow towards the ocean. Overland flow and underground flow are slower than stream flow (for equivalent rates), which reduces peak flow rates in rivers after a precipitation event. Reduction of peak flows reduces flooding and erosion. Underground flow certainly resulted in no surface erosion.

Erosion in itself is a natural process; there will ultimately be equilibrium between fine soil formation and erosion. Under natural “pristine” conditions with beaver dams the amount of fine sediments present on the land at any time was significantly higher than with current agricultural and development land use patterns. The greater amount of fine sediments contributed to greater fertility and biodiversity. Agriculture and land development currently play the major role reducing soil equilibrium amounts. The textbooks referred to this change in equilibrium as the land “wearing out”. Actually, loss of fertility may have been the result of the loss of the very fine sediments that had been captured in grasslands for eons. Current land use has so radically increased erosion that dammed ponds totally silt in a period of a few years. Research needs to be done to determine the optimal balance between wetlands and agriculture. Progressive thinking may show that sustainable agricultural production and environmentally sensitive land management practices can be achieved with the same land usage practices. The current understanding of the benefits of wetlands and the basic concepts reviewed here should cause us to seriously reconsider the positive effects of beaver dams on ecosystems.

There was an inherent conflict between early agriculture and beavers. The fertile land flooded by beaver dams was prime farmland. The beaver fashion hat industry may have developed as a by-product of the early efforts to clear agricultural land in Europe. Most of the early the fur trade, led initially by the French voyagers, the North West Company, and the British Hudson’s Bay Company, drove settlement of North America. The beaver pelt was one of the most valuable furs, leading to virtual extinction of beavers in the early 1900’s. From a historical perspective it is interesting to note that the greatest harvest rates of beaver pelts in the Lake Superior region occurred prior to the signing of the United States Declaration of Independence. The few beavers that were left when land was homesteaded were likely removed since they were a hindrance to farming. Later government agricultural drainage programs went even further to reduce wetlands. Modern agricultural drainage programs may have had less effect on wetland reduction than the earlier removal of the beavers.

Another current area of conflict with beavers is that they tend to preferentially built dams that interfere with road crossings over flowing water: they especially tend to plug up culverts (if you have an original picture of this send it and we will post it with an illustration credit). The reason for this is that the designs for road crossings tend to constrict the flow which speeds up the water, and tends to make riffling sounds. The sounds of flowing water in addition to a velocity threshold compel beavers to build dams. Clemson University has developed a correction for this problem with the “Clemson Pond Leveler.” This device is designed to quiet the sound of water and to reduce the directional velocity. A long term approach to this problem is to “just stop” constricting streams. Multiple box culverts and bridges are less constricting than single round culverts. Streams should be wider, deeper, and slower at road crossings. Our highway and waterway engineers need to be taught that constricting streams will inevitably lead to beaver problems (and associated costs). The potential for Beaver dam problems should be considered in all water project environmental impact statements and benefit cost analysis. It may be cheaper to just kill beavers, but it is more socially appealing to reduce the potential for beaver problems in the design phase of highway projects.

There is currently a debate going on over what to do with silted in ponds. The two sides of the debate seem to be to both remove the dam and restore the river to an “unobstructed” state or to dredge the sediments out of the pond. It is unfortunate that the ponds have sedimented in so quickly! Total removal of the dam would result in the captured sediments being washed away resulting in years of very high sediment loading downstream. Removing the excess sediment would be expensive, since the pond will just silt back in. Erosion preventative land use practices and stilling sediment catch basins may be a partial solution. The natural model would give some insights. In some cases the beavers continued to raise the pool level, in other cases they would leave and build upstream or downstream. The high sediment loading rates add a complex dimension to this problem. Even so, environmental decision makers must realize that the flat beaver meadows areas left as basins silt in are a natural phenomena and these may provide excellent park and recreation opportunities. The stream will still flow through the beaver meadow, but the dam forms a natural energy dissipating drop structure. This grassy meadow will flood during high flows, and will continue to capture sediments. The elevated water table caused by the meadow will still contribute to charging the lower stream during periods of drought. The full subsurface reserve would still exist and the silted in pond volume will now be part of the subsurface reserve. The exact hydrology of this system varies, but beaver dams and meadows always increase the subsurface water charge. This concept is shown in the following illustration.

Silted in Pond Becomes Meadow with Continued Subsurface Water Reserve

Beaver meadows offer same subsurface water advantages as beaver dams.

A silted in beaver pond (beaver meadow) continues groundwater storage benefits. As with a beaver dam the stream below the meadow will continue to be fed with cool ground water.

Mature beaver pond which will soon become meadow.