The illustration below depicts how beaver dams stabilize stream flow rates. The illustration shows a horizontally compressed watershed cross section between two streams, and how groundwater charge keeps the streams flowing. The stream channels are the green “U” shapes and the water flows “out of the page” towards you. Groundwater charge is the reason streams continue to flow without rainfall. With no rainfall, groundwater will continue to fill the stream until the groundwater is at the level of the black triangles. When it rains, water infiltrates to the groundwater, and raises the water table. The top half of the illustration shows the ground surface profile, and typical groundwater levels with no beaver dam. The channels can be miles apart, so the groundwater profile is also miles wide.
The bottom half of the illustration shows the elevation of the groundwater table under typical conditions with a beaver dam present. The aquifer will continue to feed the downstream side of the dam until the level of the black arrows is reached. Notice that the typical groundwater reserve is greater. This water reserve can be thought of as an “underground lake”, keeping the stream flowing with no rainfall. When water fills to the top of the dam, groundwater will be at the “full dam reserve” level as depicted in the illustration. The increased groundwater level reaches all the way across the cross section to the next watershed, and upstream as far as the pool level is raised. The “underground lake” for a 5′ high dam with a 10 mile watershed width would be 5′ high by 10 miles wide! This volume dwarfs the volume of the pond itself. The increased groundwater charge caused by beaver dams is of significant benefit to wildlife, fish, and vegetation especially during periods of drought. The elevated water table can increase vegetation miles from the dam.
As stated, the volume of groundwater greatly exceeds the volume of the pond so the thermal mass of the groundwater volume greatly exceeds the thermal mass of the pond. With a beaver dam, streams flow at more moderate temperatures. The surface of water in the pond may be warmer, but as water flows from the cool charged aquifer into the stream the temperatures are moderated. Cool springs percolate into the pond and stream bed from the edges and from below.
