I have often thought grimly of sediment load after a fire. It’s nice to see it studied.

Beaver Ponds: How do Post-Fire Sediment and Carbon Dynamics Contribute to Watershed Resilience?

In 2020 approximately 30,000 acres of Rocky Mountain National Park (RMNP) burned in two wildfires: East Troublesome and Cameron Peak fires. Fires on a landscape have many effects, one of which is facilitating carbon and sediment movement. When fire sweeps through a landscape, burning the vegetation, the ground surface is prone to erosion, slides, etc. depending on the terrain. Increased carbon and sediment in water bodies is a concern because it can impact drinking water, infrastructure, and aquatic habitat. The retention of sediment and carbon by beaver dams can dampen potential impacts to drinking water and aquatic habitat. Immediately following a fire is the time when the most sediment enters rivers and creeks. Knowing that the sediments would travel down towards the river valleys, Dunn decided to study the impacts of the fires on the sediment retention in beaver ponds within and outside of burned areas to understand whether beaver ponds impact the ability of a watershed to recover after disturbance such as fire (Figure 1). The ability of a watershed to recover after a disturbance is known as watershed resilience. As part of her graduate program at Colorado State University, Sarah Dunn researched whether beaver ponds increase sediment and carbon storage after fire.

To determine how beaver ponds affect the resilience of watersheds, Dunn set out to perform a research project to answer the following three questions: Do burned ponds store greater relative volumes of sediment compared to unburned ponds?, Do post-fire sedimentation rates in burned ponds exceed pre-fire and unburned sedimentation rates?, and Is post-fire sediment stored in beaver ponds coarser and have a higher abundance of organic carbon relative to pre-fire sediment? The rate and volume of sediment, carbon storage, and sediment grain size are expected to increase after a fire during the period when vegetation is recovering (Figure 2).

So what do you think? Will there be more sediment trapped by beaver dams after the fire? Will carbon be greater?

For the beaver ponds that were sampled and analyzed for this research project, about half were located within a burned area and the other half located in an unburned area. The average sediment volume stored in the ponds was 796 m³ and sediment volumes ranged from 4 m³ (about half a dump truck load) to 7,888 m³ (about 500 dump trucks worth). Sarah determined that burned area beaver ponds stored higher relative volumes of sediment. The sedimentation rates after fires were significantly different from sedimentation rates before the fire (Figure 3). In fact, the sedimentation rates in ponds post-fire were an order of magnitude higher than pre-fire rates in ponds. In analyzing the grain size of sediments in burned and unburned beaver ponds, Sarah found that the grain size and amount of organic carbon did not differ significantly.

Hmm more sediment but not more carbon. How does that work exactly. Wouldn’t all those burned trees be trapping carbon and when the ash runs into the soil get trapped by beaver ponds? No because what we get from burning is only a little bit carbon. And lots of other stuff “Typically, wood ash contains the following major elements: Carbon (C) — 5–30%. Calcium (Ca) — 7–33% Potassium (K) — 3–10%.’ Most of that carbon is released into the atmosphere when the tree burns.

Beaver ponds still hold onto that stuff.

Beaver ponds, which once were abundant on the RMNP landscape and are now less common, drastically changing the hydrology of the environment. Beaver dams (Figure 4) create upstream ponds that store sediment and carbon. Due to the effects, mimicking beaver dams with structures is a strategy under consideration for fire-prone areas. The research that Sarah conducted provides evidence that beaver ponds effectively trap and retain post-fire sediment. The increased sediment in beaver ponds post-fire builds on the existing knowledge that beaver ponds provide to the ecosystem is evident through the increase of pooling water and decreased water flow (Figure 4). These findings contribute to the park’s efforts to restore beaver habitat.