The illustration below 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 “out of the page” towards you. Groundwater charge is the reason streams continue to flow without inputs such as rainfall. Water will continue to fill the stream from the reserve until the downstream level below the dam is reached (black triangle). The top half of the illustration shows the surface profile, and the groundwater levels for typical rainfall conditions with no beaver dam.
The bottom half of the 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 downstream 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 storing water. This water reserve can be thought of as an “underground lake”, keeping the stream flowing. 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 flowing from the ground are the result of water flowing out of charged aquifers. These springs also flow from the stream bed. They tend to be moderated in temperature to the average seasonal ground temperature.
The volume of groundwater greatly exceeds the volume of the pond. Also the thermal mass of the groundwater greatly exceeds the thermal mass of the pond. So, with a beaver dam, streams keep flowing longer 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 edges and bottom the temperatures are moderated. Cool springs percolate into the pond and stream bed from below.