Written by: Bob Mallard
With the exception of small freestone streams, no type of aquatic habitat is more identified with brook trout than beaver ponds. In areas such as interior Maine, they are more important to the brook trout angler than freestone streams, which are often nearly unfishable due to downed timber and dense streamside vegetation.
The image of brook trout sipping insects off the surface of a dead-tree-dotted beaver pond is burned into the minds of many anglers. Located anywhere brook trout and beavers coexist, regardless of the origin of either, these unique aquatic micro-environments can be found throughout most of native brook trout range, as well as in much of the rest of the country.
Having evolved over thousands of years, the relationship between beavers and brook trout is complex and often misunderstood. Their immediate impact on brook trout can be positive or negative but is often temporary. Sometimes, however, the impacts span generations and change from one year to the next.
So-called beaver ponds are created when beavers dam up streams. They make dams out of rocks, mud, and branches, cutting entire trees down with their teeth to get the latter. The ponds created by these dams serve as locations for their homes, aptly named beaver houses or lodges, as well as protective refuge.
Beaver dams run from a few feet long to hundreds of feet. The longest documented beaver dam is in Wood Buffalo National Park in Alberta, Canada, and measures roughly 2,800 feet long. The dam was built sometime after 1975 according to satellite photographs and was the result of linking two older dams. Google Earth images show that new dams are being built that could increase its length even more. A beaver dam of nearly 2,150 feet long, almost 15 feet high, and over 20 feet thick was documented in Three Forks, Montana.
When beavers are actively maintaining a dam, it gets longer and higher, backing up more water behind it. Breaches in dams are sensed by the beavers due to changes in water level and are addressed as soon as they feel safe doing so, usually after dark. However, as soon as beavers stop maintaining a dam, it begins to deteriorate. High water flows over the dam, eroding it, and dropping the water level of the pond.
Whether a beaver pond lasts months, years, or even decades has a lot to do with local weather patterns and the area’s ability to absorb runoff from rain and snowmelt. In high-gradient areas with heavy runoff and limited buffering capacity, many beaver dams get washed out in the spring. In low-gradient areas, areas with limited snowpack, and those with an ability to absorb runoff, they can last for decades.
The positive impacts beavers have on brook trout include creating critical deep-water habitat and thermal refuge used during periods of drought and warm weather. I have seen times when practically every brook trout in a section of stream had moved into a beaver pond to escape low or warm water. They create winter refuge in streams that would otherwise freeze nearly solid as well.
While not all beaver ponds contain brook trout (if the stream does not, the pond will not), those that do allow brook trout to attain sizes and densities not normally seen in small-stream habitat. And while they can silt up and submerge brook trout spawning habitat under stagnant water, they provide important woody debris needed for spawning downstream and create rearing habitat for juvenile fish.
Beaver ponds can also increase nutrients and enhance forage, especially insects. They help reduce flooding, decrease erosion, and increase stream flow during dry periods by storing water and raising groundwater tables. They are also said to help remove sediments and pollutants such as herbicides and pesticides from waterways.
Beavers can negatively affect brook trout as well, albeit usually temporarily. Beaver dams can block fish passage, isolate populations, cause siltation, and increase predation by mammal and avian predators. As they dry up they can become dangerously, and even fatally, warm. While some problem dams last for only a few years, some, especially those in low-gradient areas, can remain in place for decades. And while blamed for giardiasis, or the dreaded “beaver fever,” the parasite that causes the malady, Giardia lamblia, is carried by birds, humans, and other animals as well as beavers.
Beaver ponds are considered by many anglers to be the Holy Grail of brook trout fishing. Finding a previously undiscovered beaver pond full of outsized brook trout is like winning the lottery. Once discovered, the location of a beaver pond is guarded like Jack Daniels guards its bourbon recipe. Writing a book about brook trout and not talking about beaver ponds would be like writing about baseball parks and not talking about Yankee Stadium.
From an angling standpoint, beavers can create some of the finest backcountry fly fishing you will ever encounter. I have caught brook trout up to 16 inches in tiny beaver ponds on streams that rarely produced fish over 6 inches. I have caught dozens of small brookies in a section of stream that prior to impounding would have yielded just a few fish.
For years my home water was a meadow stream with a series of small beaver ponds in its headwaters. Some of the dams were decades old, and others came and went from one year to the next. The farther you pushed upstream, the better the fishing got. The biggest brook trout I ever caught there was 14 inches and taken from a pool-sized rectangular pond created by three converging beaver dams.
I once stopped to fish a roadside beaver pond that had been blocked to vehicular traffic for a couple of years, the gate being opened just days before. Its roadside location resulted in a high level of angler exploitation, limiting it to just freshly stocked fish and a few small wild fish. I caught 10 beautifully colored, deep-bellied 12-inch brook trout, a testimony to how fertile, yet easily exploited, these waters can be.
Fishing beaver ponds is not easy. They are often hard to find and hard to get into when you do. The area surrounding beaver ponds is often wet, rutted, and littered with hummocks, brush, and other things that make getting around difficult. The standing dead trees and submerged logs and branches present a challenge to the fly fisher, as they are a major source of snags. And rarely do you have enough room to get a good backcast.
Most beaver ponds are either too muddy or too deep to wade. Casting from the shore is difficult and limiting. The best way to fish beaver ponds is with a doughnut-style float tube. Add waders and rubber lug–sole boots sans fins, and off you go walking when you can and kicking when you can’t. Due to the size of these waters, you can’t get blown off course and unable to kick your way back.
When I was young we found most beaver ponds by accident, usually while hiking, hunting, or canoeing. While we used topographic maps to identify likely spots, we were wrong more often than we were right. Today’s fly fisher has the benefit of tools like Google Earth that can be used to scan the countryside looking for these brook trout utopias. Using data culled from these maps and a handheld GPS, waters that would have been difficult to find just a decade or so ago are easy to find today.
Beaver ponds can appear out of nowhere and disappear just as quickly. They are at their best a year or two after being formed, and most do not hold up for long due to angler exploitation or Mother Nature. By the time a visible trail is cut to a beaver pond, it is usually too late, at least for fishing.
Beaver ponds are beautiful and bountiful. They are rich in aquatic, mammalian, and avian life. They are home to fish, frogs, salamanders, turtles, waterfowl, wading birds, muskrat, otter, mink, and beaver, and visited by moose, deer, and bear. Beaver ponds are small, quiet, and a great way to enjoy a day of fishing for brook trout.
Beaver ponds have a certain mystique to them. The fish are often larger than those found in natural ponds of similar size. Unfortunately, as part of a stream, beaver ponds rarely have the benefit of protective regulations. This makes them easy to exploit, and when that happens it affects everything involved, including us, as once the trout are gone they can take years to repopulate. Knowing that what you just found is most likely temporary, there is a sense of urgency that keeps bringing you back to a beaver pond with brook trout, recognizing that while it is great today, it most likely won’t last forever.
Excerpted from Bob Mallard’s book, Squaretail: The Definitive Guide to Brook Trout and Where to Find Them. For signed first edition copies, go to www.BobMallard.com/shop/. Bob is a former fly-shop owner, author, blogger, and registered Maine guide.