Using Nature's Engineers to Slow Early Runoff

May 2021

In our last article, I teased an unlikely hero, one who can slow the premature spring thaw that we've been discussing in our Alpine Ice-Off series.

I posed the question: "Is there anything that can be done to slow the early release of spring runoff? A way to retain the water rolling down mountains without getting immediately diverted into large reservoirs, where hot summer temperatures evaporate that precious resource [water] before it's used?"

As spring runoff occurs earlier in the year, Rocky Mountain streams lose their snowy reserves held in the high alpine zone leading to decreased summer flows, increased stream temperatures, and rivers that reach base flow earlier in the season. These reduced flow conditions stress trout, impact their spawning behavior, and impact the larger river ecosystem. Earlier spring runoff events can even worsen the effects of drought and increase the risk for wildfires. Any future-thinking trout angler should be concerned with the trends in runoff we are seeing across the West.

So what can be done?

Before civil engineering ever existed, there were 'un'-qualified engineers spread across most of North America, damming rivers and shaping the landscape. You guessed it–beavers. But beavers get a bad rap sometimes. In some angling circles, beavers are thought to impede fish passage, and migration, as they build dams across stream channels. They can ruin your favorite fishing hole, and even completely change the morphology of a river and surrounding ecosystem. Depending on where they are found this might be true, but in many tributaries and streams across the West, beavers offer more benefits than drawbacks.

Let's look at the science

Several studies have shown cutthroat and brook trout have the ability to navigate beaver complexes unimpeded, while non-native trout (like brown trout) cannot similarly navigate past beaver dams. These traits are attributable to the co-evolution of brook and cutthroat trout alongside beavers in North America. This is an overlooked but critical element when considering strategies to prevent the spread of invasive trout into waters home to native trout. We already know that invasive rainbow trout out-compete and sometimes hybridize with cutthroat in the West, so the most effective mitigation strategy is often to prevent invasive trout from occupying waters home to native trout. This seems counter-intuitive when also considering the restorative push to reconnect habitat for native species from poorly designed culverts and diversion structures. But, if one-way passages that allow cutthroat trout movement while restricting invasive trout movement could be constructed without construction equipment disturbing existing stream channels, trout conservationists should applaud this eco-engineering feat (and the engineer responsible).

As for habitat loss from beaver construction, beaver dams actually diversify riverine habitat beneficial to trout, but you must consider all life stages of the trout to understand the benefits. First, fallen trees provide canopy cover which keeps streams cool. Fallen trees, when submerged, also create velocity refugia which are important for juvenile and larger trout. Next, beaver dams don't stop 100% of water flow. Instead, they disperse water, widening the streambed, creating new meandering channels. As water finds a new course, new riffles are created and water is oxygenated, to the benefit of aquatic invertebrates, amphibians, and trout.
The benefits keep going. Beaver dams retain enough water to saturate the water table and surrounding soil. This broadens the riparian footprint surrounding a stream and keeps plants greener, even in the face of drought. During hot and dry summers, the fully soaked ground wards off fuel for wildfires and offers a refuge for any animal escaping a smoldering fire. You can find images of hillslopes fully scarred from burning, and smack in the middle is a lush stream complex downstream from a beaver dam.

As for runoff

When beavers construct dams, the time required for water to enter the inlet of the system and then make its way through the outlet of the system is increased. The time spent traveling from inlet to outlet is referred to as residence time. With networks of beaver dams located higher in a watershed, the residence time for water moving downstream is increased, offering a way to slow the early pulse of spring runoff and prolonging the flow of cool water over the course of the season which even benefit river systems and larger confluences downstream.

Are there negatives?

Though beavers offer an immense array of benefits to humans and fish, there are no doubt site-specific caveats. North America is geographically diverse and beaver reintroduction may be more acceptable in certain regions. For example, western conservationists have gained more traction with beaver restoration efforts compared to the Midwest and East. Water availability in the West is much scarcer compared to the Midwest and Eastern U.S. and this fact might drive the pattern of reintroducing beavers in western streams, while in the east the focus might be on removing dams and opening fish passages.

In the Rockies, western water rights must also be considered a roadblock to widespread restoration work involving beaver dams. Though it might be in nature's best interest to slow the flow of water throughout the season, if a senior water right requires the delivery of water, the slow down process from beaver dams may be seen as a hindrance to water delivery. Stay tuned for later articles about western water rights and impacts on fish.

Beaver Reintroduction

Many scientists have identified the benefits available to both trout and humans when beavers are present in western streams. Fortunately, beavers were once widespread and can thrive in many different environments. So, now they are being reintroduced to continue their work in new watersheds. I‘ve even been able to fish for native cutthroat trout in streams rehabilitated by beavers. There are plenty of fishing opportunities, and big fish, that prove the benefits to trout from beavers. Plus, the damming that beavers offer are a low-price alternative to human-designed restoration. One study looked at the ecosystem services provided by beaver dams and quantified them in dollars/hectare. No matter the dollar amounts attached, why pay an engineer for an intensive restoration project when evolution has already trained and certified beavers. They are plenty qualified, and based on their work ethic, they seem to enjoy their day jobs.

Sources:

1. Kemp, P.S., Worthington, T.A., Langford, T.E.L., Tree, A.R.J., Gaywood, M.J., 2011. Qualitative and quantitative effects of reintroduced beavers on stream fish. Fish and Fisheries 13, 158-181.
2. Goldfarb, B. 2020. How beavers became North America's best firefighter. National Geographic. https://www.nationalgeographic.com/animals/article/beavers-firefighters-wildfires-california-oregon
3. Lotkeff R. L., Roper B. B. & Wheaton, J. M. 2013. Do Beaver Dams Impede the Movement of Trout? Transactions of the American Fisheries Society 142:1114–1125. https://www.fs.fed.us/biology/resources/pubs/feu/13Lokteff_etalBeaver.pdf
4. Goldfarb, B. 2018. Beavers, rebooted. Science. https://science.sciencemag.org/content/360/6393/1058/tab-figures-data
5. Hafen, K. 2017. To what extent might beaver dam building buffer water storage losses associated with a declining snowpack? USU Graduate Thesis. https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=7648&context=etd
6. Thompson, S. et al. 2020. Ecosystem services provided by beavers Castor spp. Mammal Review. https://onlinelibrary.wiley.com/doi/full/10.1111/mam.12220