Animals and Environments 3: Beaver of the Breaks

Ah, beaver, the cute rodent property owners love to hate. Beaver are, as one author described them, “nature’s wetland engineers,” re-working the local hydrology to suit their own needs. In the process they can have some very long-term effects on the environment around and down stream of their chosen location. The Canadian River valley, although at the margins of beavers’ usual domain, showed the effects of their activities in several different ways.

Industrious Beavers at Niagara Falls, as shown on a 1731 edition Moll's map of the British Colonies in North America. Library of Congress, Geography and Map Division.  From

Industrious Beavers at Niagara Falls, as shown on a 1731 edition Moll’s map of the British Colonies in North America. Library of Congress, Geography and Map Division. From

OK, so early European authors had a slightly inaccurate view of how beavers did their thing. They had a good reason for the confusion: European beavers live in the banks of streams instead of making ponds and lodges. But the basic idea’s pretty close.

Beaver build lodges and dams on fresh-water streams, usually in areas with ready access to the types of small trees and plants beaver prefer to eat (willow, birch, aspen). Beaver do not eat older wood per se, but do eat the inner bark and small, tender twigs and branches. Their digestive systems include bacteria to digest some of the cellulose, but they prefer bark and the cambium layer. Beavers gnaw down saplings and small trees, then drag them into the water and use them, along with sediment from the stream, to build water-resistant and water-tight structures. The pond behind the dam serves as the location for the lodge, where the beaver raise their kits. The beaver also stash food in the pond for winter, preserving and protecting it. Their dams can be very impressive and sturdy, at times requiring a considerable amount of effort and explosives to remove them from the waterway.

The beaver dam and pond has several effects on the hydrology of the stream. The largest known dam to date is one in Woods Buffalo National Park in Canada, and measures roughly 2790 feet from bank to bank. Most dams are not quit that large, but they all have the same hydrologic effects. The dam stops stream flow for a while, until the pond outgrows the dam and water overtops it, or finds a new, slower way through the mass of material. The pond raises the local water table, allowing water to seep into the surrounding soil and creating a larger wetland than was there before, and potentially favoring some kinds of plants while drowning others. The pond acts as a flood buffer, slowing the rush of water and making the rise and fall of the stream less dramatic. The stream becomes less “flashy,” less prone to rise and fall abruptly when storms come through or snow melt hits the watershed. In like manner the pond and dam keep more water in the stream during dry periods, up to a certain point. After ten months without rain, even beaver ponds won’t help a stream. The wetland and pond can act as water purification systems, allowing the slow filtration and decomposition of sediment and biological materials in the water.

Along with water, the pond and dam catch sediment. Over time, the pond fills in, becoming first a smaller pond, then a marsh, then a beaver meadow lush with grasses and forbs. When the Hispano settlers first moved into the Canadian River valley in Texas in the early 1870s, they used those old beaver meadows for hay meadows as well as setting up their acequias, their irrigation systems, there in the rich and relatively level soil. By that point, the beaver have already moved on, usually after having eaten out all their preferred trees and bushes.

As mentioned above, the Canadian River valley, especially the western section in what became Texas and in far eastern New Mexico, provided beaver with at best marginal habitat. The semi-arid climate did not favor the kinds of trees beaver prefer, and what did grow (cottonwood, hackberry, scrub oak, juniper, slippery elm) tended to be a bit sparse. Still, the valley formed a connection between the Rocky Mountains to the west and the lusher Great Plains and Low Rolling Plains to the east, and early Anglo explorers commented on the enormous beaver dam they found on one of the tributaries of the North Canadian, probably not far from modern Perryton, Texas in the northeastern corner of the Panhandle.

The Hispano residents made a few attempts to trap the beaver, but without much success. Muskrat too lived along the streams feeding the Canadian, but their structures tend to be less dramatic than those of the beaver.  The Canadian Valley, off the main fur-trade routes, didn’t see the heavy trapping so well-known to the north. I also suspect that the pelts of southern beaver would have brought lower prices on the market, but that’s just a guess. The lush under-coat of the beaver, beneath the guard hairs, brought enormous amounts of money in the hat trade, as the history of Canada and of the US fur trade attests.

The brown gold of Canada: beaver on the Parliament Building in Ottawa.

The brown gold of Canada: beaver on the Parliament Building in Ottawa.

The droughts of the 1850s-60s likely stressed the beaver population even farther. The arrival of the Hispanos and Anglo ranchers brought and end to the beaver as people cut down the few remaining trees to build with. The first Anglo residents’ accounts do not mention beaver, a telling omission. Without beaver, their dams decayed and washed out. Without the dams, the streams ran free, surging and declining, eroding their beds more heavily. This in turn contributed to changes in the Canadian River as additional sediment washed into the main stream, and the water supply from tributaries peaked and fell at greater speed. How much this contributed to the physical changes one of the Hispano residents, Jose Ynocencio Romero, recounted to a later historian remains uncertain because of other events going in the river’s “life” at the time, but it likely played at least a small role.

Like the bison on the uplands, beaver were a keystone species in the Canadian River valley. They shaped their environment, which in turn affected the larger valley’s hydrology and biology.

5 thoughts on “Animals and Environments 3: Beaver of the Breaks

  1. I have seen many more bank dens for beaver than lodges, but have noticed that this is generally dictated by terrain. If the streams or ponds in question are in hilly terrain where the beaver can dig an underwater tunnel into the bank and then within a few feet have enough soil overhead to make a ‘room’ above the high water level; then they will generally dig bank dens. If however the surrounding area is relatively flat and/or marshy they will generally build lodges. I have seen where beaver found a small rise in otherwise flat swamp and dug multiple bank dens in that small rise of ground that provided enough soil to do so, and have also seen lodges built in steep, rocky terrain where digging bank dens would be difficult. But generally speaking, if the ground is hilly and conducive to bank dens, that is what they will build, only very occasionally building ‘traditional’ beaver lodges in such areas.

    Yes southern pelts are worth considerably less money than prime northern skins.

    • I suspect if you get to detailed case studies, soil composition, topography, hydrology, and perhaps even “laziness” play a role in bank vs. lodge denning. (As the family veterinarian put it “animals, like people, are only as smart as they have to be.” Replace smart with energetic, and if a beaver can get good results from a bank den with less effort, why build a lodge?) The dams and localized tree cutting are the major environmental factors in any case.

      I’d be curious to do an under-coat comparison on modern High Plains beaver as compared to Northern Rockies beaver. I suspect the difference in pelt density is even great than it was in the 19th century because of the warmer winters compared to the 1800s.

      • I’ve got a few prime Northern Rockies beaver pelts in the freezer I need to finish. While I can’t recall ever looking at any Texas beaver pelts, I have done the entirely unscientific comparison of running my fingers through the fur and ruffling it backwards on some pelts from Louisiana. The difference is amazing and obvious to even an inexperienced eye.

        I’m sure you are entirely correct on why beavers often prefer bank dens to lodges. After all you came to the same conclusion I did, so that PROVES you are right. 🙂

  2. By the way, do you have alder down in your area? It is a preferred beaver food both on the coast (where willow, birch and aspen are not generally available) and here, where they seem to prefer it over about anything but willow. Now that I think about it I don’t recall seeing any in New Mexico and Arizona when I was down there, and don’t recall anybody I have talked to from the south ever mentioning it.

    I am very much enjoying your posts on Animals and their effects on the Environments they live in.

    • The only alders are in the far eastern Piney Woods section, where you have acid soil and plentiful rainfall. They don’t seem to get much farther west than Waco. Once you get west of Ft. Worth and the “black waxy” soil line, the soils are too alkaline, and the rain too sparse, even in otherwise wetland areas. The native wetland plants in the High Plains were/are cottonwoods, hackberry trees, and willow-like brush, plus rushes, sedges, arrowleaf, and the usual sorts of wet-footed plains plants.

      Thank you!

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