'We need to understand the trade-offs and benefits'
As ecosystem engineers, beavers build resilience into the landscape.
Above ground, we can see changes wrought by beaver ponds such as increases in biodiversity and water retention. But UConn Department of Earth Sciences researcher Lijing Wang says we have a limited understanding of how they impact what happens beneath the ground. In research published in Water Resource Research, Wang and co-authors study how water moves through the soils and subsurface environment and detail new insights into how beaver ponds impact groundwater.
Groundwater can be an important source of water for streams, especially late in a dry summer, it may be the only source of water sustaining a stream, says Wang, and researchers are interested in understanding if and how beaver ponds impact groundwater as these details are important to consider for water management and restoration efforts.
Wang explains that some initiatives have included building beaver dam analogs to mimic what live beavers do and these man-made structures similarly extend the wetland and make an area more drought and wildfire resilient, however there are no comprehensive studies that focus on understanding beaver-induced changes to the subsurface water.
"Our work here develops one of the first hydrologic models that helps us understand what happens from the beaver inundation to the subsurface system under different subsurface structures," says Wang.
For the study, the team used both in situ measurements including geophysical surveys, hydrologic data and modeling, along with a machine learning method called neural density estimator, first adopted in astrophysics, to calibrate the model to therefore better predict the changes happening due to the presence of beavers.
"We use the model-data integration method to try to make our model replicate what happens in our observed data," says Wang. "Now that the model is calibrated to real-world observations, we can then better understand how each control works."
Wang explains that one example controlling the flow of water includes different subsurface structures. The region of the Rocky Mountains where this study focuses is home to rivers with gravel bed systems, composed of large cobbles/gravels that extend as deep as 16 meters and outward from the river into the floodplain. They explored different combinations of subsurface structures in their models and found that characterizing the floodplain structure is critical for understanding the ways beaver ponds impact the movement of water in the subsurface environment. Wang says with shallow gravel bed and soil layers, beaver ponds may have a greater influence on how groundwater is replenished or recharged.
The researchers also explored the movement of water available for evaporation to the atmosphere, or evapotranspiration (ET).
"Evapotranspiration is particularly important in water-limited region like the U.S. West, where if there is more water in the floodplain then more water is evaporated to the air," says Wang. "Thinking about the water budget, beaver-induced inundation may reduce how much water is in the system where a lot of water evaporates. Our analysis found if the soil structure on top of the gravel bed is very thick, then the ET could be large enough that the recharge could become lower or even less than without beaver ponds, because more water is used by the air and vegetation."
In all, Wang's team found that beaver ponds increased groundwater recharge 10 times compared to a dry period. The question they are currently trying to answer: where does that water go next?
"Our results show that when the water reached the gravel bed, it does not stay there, it goes downstream. Thinking of the gravel bed as 'a thick river' underneath the stream bed, there's more water flushed downstream in the subsurface than we thought. It's not staying there and sustaining the local water table," says Wang.
Though this research focuses on beaver ponds in Colorado, Wang says she is starting to focus on New England, and she has started monitoring local beaver ponds.
"In New England, we have different problems compared to the Rocky Mountains, where they have a relatively simpler river network. In New England, we have complex river networks with more tributaries, channels, and beaver dams, which give us more biodiversity, and sustains mature floodplains and wetlands overall."
Understanding the intricacies between land use practice and its subsurface environment is critical for understanding exactly how beaver ponds will influence other aspects that we may not immediately come to mind, Wang explains, such as potentially negative changes to water quality and for this we need a comprehensive analysis.
As beavers slow the flow of water and create ponds, this changes the subsurface oxygen conditions and leads to lower oxygen, or anoxic, concentrations in the water. These conditions can then lead to the proliferation of anaerobic bacteria, whose activities in the sediment can mobilize heavy metals that would remain trapped in more oxygen-rich conditions. For these scenarios, location and history are key.
"If you are in a pristine area with no previous industrial activity that may not be a huge problem," says Wang. "However, if you are in the area like our site in Colorado that is near abandoned mines, we can see more soluble metals downstream. Beaver ponds can increase the ecological benefits, but we lack a comprehensive understanding of water budget and water quality. We need to understand the trade-offs and benefits."
