Beaver Dam Information Site
This site is dedicated to one of the primary keystone species, the beaver. A keystone species is one that modifies the natural environment in such a way that the overall ecosystem builds upon the change. The ponds, wetlands, and meadows formed by beaver dams increases bio-diversity and improves overall environmental quality. It is our opinion that many environmental decision makers do not fully understand the positive effects that beavers and dams bring to ecosystems. This is understandable, because beavers had been virtually eradicated prior to the development of modern scientific methods. This site incorporates first principle engineering concepts in combination with environmental observations to illustrate the extent that our watersheds have changed with the removal of beavers. Beavers affected our ecosystems and land in a very extensive and positive way. Modern society has recently begun to realize the benefits of wetlands. This realization marks a turning point in over 300 years of extensive wetland eradication. Beaver dams are the primary natural method of establishing wetlands. Beaver dams represent the only natural methods of forming lakes, ponds, and wetlands in most watersheds. The exceptions to this would be glacial lakes, or lakes formed by geologic activity. This website is designed to show the numerous benefits of beaver dams.
Benefits of 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 beaver meadows form
· Stills and deepens waters, improves canoeing.
Causes of and Effects of Wetland Removal
Most blame agricultural drainage and land development as the primary reasons for wetland loss. We do not think about the removal of beavers because we have no modern experience with this effect. Modern agricultural drainage may have less effect of wetland reduction, than the original removal of the beavers. Land 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 can see a ditch, but cannot see the absence of a beaver dam. 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.” This paper can be downloaded from the link http://www.wetlands-initiative.org/TWIpublicationsWetlandsFlooding.html This paper states that watershed policies of agricultural and urban drainage 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. Of course, it would be impossible to restore all of the wetlands, but the benefits of wetlands should be considered when choices are available.
Stream Bank Erosion and Stream Sediment Loading
specific example of the missed opportunity of beaver dams is in 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. Currently, these eroding banks are far from
achieving a state of “equilibrium” and will continue to scour both deeper and
wider. One alternative method to stop
stream bank erosion and meandering would be to restore beaver dams in these
erosive meandering areas. The photograph
to the right shows an unstable stream bank about 6 feet high. The width of the channel is 20 feet. Tree roots and vegetation are temporarily
maintaining the unstable high angle of repose.
This condition is not stable. The
topography of the ravine in which this stream flows is a flat 200 yard wide
meadow between steeper ravine side slopes.
It is apparent that the stream channel is gouging deeper into the
meadow. The sediments from the bank
erosion will be washed downstream, ultimately into the
The new beaver dam in the picture to the left is about 3 feet high. Repeating the previous paragraph the location of this dam is below the unstable stream bank area 150 yards upstream. This dam triples the upstream depth compared to the downstream depth. Any increase in width or depth of a stream channel (cross section area) will reduce the stream velocity in proportion to the increase in width times depth. Upstream of the dam, sediments are being trapped because of the reduced velocity. The upstream area will silt in and if the beavers are left undisturbed, the dam will continue to be raised until it actually tops the channel bank and will be built wider – beyond the scoured existing banks. The sediment stilling effect becomes more pronounced as the pond gets wider. Ultimately, all of the erosion potential of the previous photograph would be stopped. The meadow and wetland would be restored! Rather than having several hundred yards of eroding stream bank loading the stream with sediments, there will be a single dam slowing the water, stilling the sediments, and dissipating the erosive energy. A part of one the original ancient dams that formed this meadow still exists in this area, it is 12 feet high with a base width of 30 feet. It is located at a point where the ravine width narrowed to about 100 yards.
Several questions in emails have been raised regarding habitat conflicts between beaver dams and fish. If the area depicted above this dam is restored to a wetland/meadow will it be suitable for the same types of fish? The answer is that our notions of natural stream channel profiles are incorrect. It is necessary to recognize the scouring/deepening channel in the stream bank photograph above as unnatural. If the ultimate outcome for the floor of this ravine area is a pond or wetland, there will be a change in the habitat. The pond will be suitable for some types of fish depending of the sediment, nutrient, and pesticide loading levels. The benefits of wetlands and meadows caused by beaver dams are typically seen downstream. Wetland buffers upstream of lakes, for example, improve lake water quality by reducing sediment and nutrient loading into the lake.
Beaver Dam Effects on Watershed Subsurface Water Reserve
The illustration to the right 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. Water springs are the result of water flowing out of charged aquifers. These springs can occur above and below the stream surface. They tend to be moderate in temperature at the average seasonal temperature.
covered most of
In the 1805 Lewis and Clark expedition up the
Beaver Dams and Fish
Beaver dams pose no unnatural hindrance to fish and may actually beneficial to such native cold water fish as trout. Beaver dams were the norm prior to 1700 in
Landscape Differences with Beaver Dams
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, reverse soil compaction improving permeability and rooting conditions for plants. 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.
Conflicts with Beavers
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
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.
Benefits of Beaver Meadows
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 either 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 upstream 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 meadow areas left after pools silt in are natural phenomena and these may provide excellent park and recreation opportunities. The stream will 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 illustration to the right.
Pond above a mature beaver dam, nearly silted in, will become a meadow when the beavers leave.
The following links substantiate the ideas presented on this page. If you want your page linked here please write!
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The ideas on this page originated from coursework in
the Agricultural Engineering Department at the
Last Updated: 18 September 2008