International Journal of Molecular Ecology and Conservation, 2025, Vol.15, No.5, 217-228 http://ecoevopublisher.com/index.php/ijmec 22 1 subsurface flow (soil flow) path of the river: the underground runoff that originally flowed downstream along the river channel is changed to seep laterally, replenishing the underground aquifers on both banks. This process has raised the groundwater level in the riverbank zone. Monitoring in the tundra region of Alaska has shown that beaver DAMS can raise the groundwater level within several hundred meters downstream by 10 to 15 cm, significantly widening the moist zone along the river (Tape et al., 2022). In addition, beaver DAMS often have overflow over the top or openings on the side of the dam, with small streams of water flowing around the dam, forming curved floodplain wetlands. These slow seepage further promote the infiltration of surface water into the ground. Therefore, the areas where beavers build DAMS exhibit a stronger "water conservation" function, increasing the exchange frequency and volume between surface water and groundwater (Oleszczuk et al., 2024). The benign exchange of groundwater and surface water has multiple benefits for the ecosystem: On the one hand, the increased groundwater reserves can recharge the river flow during the dry season and maintain the continuous flow of streams (Thompson et al., 2021); On the other hand, the water seeping into the ground stays in the aquifer for a longer time, which is conducive to the decomposition and removal of pollutants through microbial action (such as the removal of nitrates through denitrification). A review study by the EPA found that nitrate nitrogen in the Beaver Dam area decreased significantly after permeation through the soil, as water retention and groundwater exchange provided sufficient time for denitrifying bacteria to act (Grudzinski et al., 2022). In addition, groundwater exchange also stabilizes water temperature: warm surface water seeps into the ground during the day and slowly flows back into the river at night, which helps to reduce the fluctuation of water temperature between day and night (Majerova et al., 2015). In a study conducted in Colorado, USA, the strong groundwater gradient caused by beaver DAMS was more than ten times higher than the seasonal hydrological fluctuations. The soil flow driven by this gradient effectively removed approximately 44% of nitrate nitrogen (Dewey et al., 2022). 3.3 Dynamic changes in water quality and sediment Building beaver dams strongly changes water quality and sediment movement. On the one hand, dams trap sediment and reduce the amount carried downstream. When the flow slows in front of the dam, more than 90% of suspended particles settle in the pond, so the water below the dam becomes clearer (Ecke et al., 2017). Studies in different climate zones confirm this. Downstream of dams, water usually shows lower turbidity and fewer suspended solids compared with upstream areas without dams. This “sediment trap” effect also lowers silt build-up in the lower channel and helps keep the riverbed stable. On the other hand, beaver wetlands filter nutrients. Plants and sediments in the pond can absorb nitrogen and phosphorus, reducing their loss downstream. A global review of 267 studies shows that nitrate levels below dams drop significantly, while total nitrogen and total phosphorus change little or fall slightly (Grudzinski et al., 2022). The slower current and larger contact between water and land also favor nutrient removal (Cooper et al., 2025). But wetlands created by beavers are rich in organic matter and often lack oxygen, which may bring some drawbacks. For example, dissolved organic carbon and ammonia nitrogen can rise, and microbes may turn mercury in the mud into toxic methylmercury. The US Environmental Protection Agency reports that methylmercury and dissolved organic carbon often increase slightly downstream of dams, linked to microbial activity in low-oxygen wetland conditions (Grudzinski et al., 2022). In addition, dam pond sludge can release methane through anaerobic fermentation. Some researchers worry that large beaver wetlands may boost greenhouse gas emissions. Yet field data from cold regions show that methane increases are small and do not cancel out the ecological gains in carbon storage and water quality improvement (Fairfax and Westbrook, 2024). 4 The Promoting Effect of Beaver Activities on Biodiversity 4.1 Responses of aquatic and semi-aquatic biological populations Beaver dam construction causes the river environment to change from turbulent to gentle, providing a new suitable habitat for various aquatic and amphibian organisms, which is usually conducive to improving the diversity and abundance of aquatic organisms. Most amphibians benefit from the still water habitats created by beavers, and the community richness increases, which makes beaver ponds regarded as the key "microhabitat" for maintaining regional amphibian diversity (Dalbeck et al., 2020). The response of fish communities to beavers
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