May

Cow Creek: Alternative Rhythms

The trail to Bridal Veil Falls starts gently. Unlike many trails in the park, this one doesn’t force me to immediately start scaling the Rocky Mountains (or so some trails can feel). The trailhead is located at the McGraw Ranch, originally homesteaded in 1875 and used primarily as a dude ranch until acquired by the park service in 1988. The cluster of buildings, along with the name Cow Creek, reminds me of the park’s recent history and of the land uses that have shaped this region.

By May the creek is running high with snowmelt. Patches of old, crusted snow persist in the deep shade beneath the conifers along the creek, even as the first flowers are starting to bloom on the south-facing slopes above. The trail follows the creek upstream through a broad, open valley. The terrain appears as though it could have been sculpted by Pleistocene glaciers, as in Wild Basin or Moraine Park, but glacial ice never filled this valley. What this valley does have in common with Wild Basin, the Upper Colorado River, Moraine Park, and Lily Lake is that the appearance of the valley today reflects the activities of beavers.

I am here on a warm, sunny day in mid-May to map the beaver dams along the creek. The rhythm of my year is accelerating for the strenuous physical activity of summer field research. While mapping logjams the previous summer, I realized that many of the breached soil berms across the creeks I walked were the remnants of old beaver dams. Intrigued at the large number of these old dams throughout the park, I decided to systematically map them and try to understand whatever story they might tell. Cow Creek has abandoned dams spaced so closely along the valley that the stream must have been mostly ponded water during the era of maximum beaver occupancy.

Ecosystem Engineers

Beavers have received many admiring names because of their skill in building dams: natural engineers, Nature’s engineers, and ecosystem engineers. Ecosystem engineer can refer to any plant or animal species that physically modifies its surroundings to create habitat for itself and, inadvertently, for other organisms. Beavers are usually the first species that comes to mind as an ecosystem engineer because of their superlative skill in creating their own habitat by ponding water behind dams.

I listen to Cow Creek flowing swiftly through a channel largely unimpeded by beaver dams. A few older dams overgrown with grasses and woody shrubs remain, but mostly the water moves without interruption through the abandoned beaver meadow. If a beaver stood beside me today hearing what I hear, things would be different.

The sound of running water can trigger an innate urge in beavers to build dams. Anything that comes to hand—or paw—is used to satisfy this urge. I have seen beaver dams that incorporate bowling ball–sized cobbles and a beaver dam that included a full rack of moose antlers. Mostly, dams consist of chewed pieces of wood about the thickness of a finger to a forearm, with plenty of silt and clay plastered among the interwoven pieces of wood. The dam represents a continual work in progress that the beavers must maintain against the relentless movement of water downslope. Sometimes the beavers choose a secondary channel or a tributary entering the side of a valley and build dams that create a pond perched several feet above the main valley floor. Elsewhere, the animals build one dam after another across a stream so that the valley bottom resembles a staircase of beaver ponds.

The work of building and maintaining the dams falls first to the adult female of the colony. The male and the yearlings help her with these tasks, but she is the primary engineer. Knowing this, it amuses me to read nineteenth-century natural history accounts describing the work of “Mr. Beaver” in building the family home. Mostly the dams are only 2 to 5 feet tall, but they can be extremely long. The largest dams can be seen on satellite imagery, such as one dam 2,800 feet in length in Canada’s Wood Buffalo National Park.

In walking across valley bottoms occupied by beavers I have frequently stumbled into the other primary engineering feature that the animals create: canals one-beaver-wide that can be difficult to see among tall grasses and sedges, but are sufficiently deep to thoroughly soak my foot and leg. Despite their ability to waddle about on their hind legs with an armful of mud and wood clasped to the chest while working on a dam, beavers are much more swift and agile in water than on land. Consequently, they dig a network of canals to make it easier to find food and building materials and escape from predators. Beavers typically have underwater entrances to their lodges and bank dens, and the animals excavate holes to maintain air exchange between the den, entrance burrows, and the surface. These holes are difficult to see when I walk in a beaver meadow but are exceptionally effective at trapping my foot.

I know of no other animal—except humans—that so thoroughly modifies the surroundings. Because I work in rivers, the more I learn about beavers, the more fascinated I become. I am succumbing to what Arthur Radclyffe Dugmore in 1914 called “the romance of the beaver” in a book of the same title. I have plenty of company. Descriptions of the animals and their engineering go back to Herodotus, Hippocrates, and Pliny in the ancient world, and scientists today sing the praises of beavers as architects of biodiversity.

The phrase beaver meadow was first introduced to scientific writing in a 1938 paper in the journal Science, but the name had been in use for many years among people living in New York and New England, and for centuries in England, where “beaver lea” gave rise to the place name (and woman’s name) Beverly. A beaver meadow is a maze of new ponds, old ponds gradually filling with sediment, wet meadows that grew where ponds completely filled, and multiple stream channels and beaver canals winding in and out among dams that also wind across the valley bottom in complicated patterns. To walk into an active beaver meadow is to become quickly lost among thickets of densely growing willow, river birch, and aspen. The spatial pattern of the landscape is best understood when seen from above, in an aerial photograph or from the perspective of a nearby, steep-sided ridge.

Not many people willingly walk into such a wetlands because of the difficulty in moving about. I do, because I cannot effectively see important details from a distance. Mapping dams within a beaver meadow is a research task that leaves me grateful for trails. Even chest waders provide only limited protection. Sinking into the organic-rich ooze of the pond bottom remains too easy, as does getting firmly stuck while trying to squeeze among the innumerable willow stems growing on the berm created by a dam. The beaver ponds along Cow Creek can be comfortably viewed from the trail, but the narrowness of the beaver meadow here partly reflects the history of cattle grazing in this valley. Cows like riverside vegetation just as much as beavers do. Although cattle no longer graze here, the effects of their presence linger in the form of grasses rather than willows along the creek.

The more I learn about the history of human manipulation of seemingly natural landscapes, the more suspicious I become that the appearance of any stream I study at least partly reflects past human activity. During the 1950s, for example, the US Forest Service used aerial herbicide spraying to limit woody plants in meadows grazed by cattle. Although the herbicides have not been applied for half a century, some of these creek-side meadows today remain grasslands rather than willow thickets. Even in the absence of herbicides, large numbers of cattle grazing riverside vegetation for years at a time can kill willows and other woody plants by repeatedly eating the plants down to the ground. Cattle have not grazed the beaver meadow along Cow Creek for more than twenty years, but willows and aspen are just starting to recover and the scarcity of these trees probably limits the number of beavers that can survive in this valley.

One beaver can chew down as many as 300 small to medium-sized trees each year. Beavers dine on salads in the summer, when up to half of their diet can consist of forbs and grasses, but the remainder of their food comes from bark and twigs. An adult needs somewhere between 1.5 and 4 pounds of this food daily. A colony of beavers typically includes a mated pair of adults, their yearlings from the previous year, and the kits of the year, which can work out to anywhere from six to twelve animals. Along a favorable stream, colonies can be spaced at just over half a mile apart. Competition for food in the form of intensive grazing by cows, moose, or elk can limit beaver populations and the extent of beaver meadows, a situation that is widespread within Rocky Mountain National Park. As I hike the trail along Cow Creek, I see a beaver meadow that has fallen on hard times, with limited width across the valley bottom and relatively few active dams.

I pause to watch a buck mule deer lying down and chewing his cud in the meadow. The deer’s cheeks bulge as though stuffed to bursting and he seems unfazed by my presence. A little farther on, a coyote forages in the meadow. When I first catch sight of it, the coyote is standing head down, wagging its tail like a dog. The animal eats something and continues nosing about in the grass. I admire the coyote’s fur, still thick from the winter recently past. As the coyote looks up and sees me, it becomes alert, but not really worried. I resume hiking and the coyote moves slowly into the forest.

Ghosts of Beavers Past

Three miles in, the trail up Cow Creek starts to climb the northern side of the valley toward the topographic exclamation point of Bridal Veil Falls. The trail ends at the falls, but I easily continue upstream along the creek. The small creek is narrowly constrained above the falls, flowing along a course punctuated by logjams and breached, vegetated berms across the creek. These berms are old beaver dams, recognizable as a linear mound perpendicular to stream flow, with grasses and clumps of willows growing from the mound. Sometimes the ends of beaver-gnawed sticks protrude from the mound, particularly where the creek cut through the dam once the beavers abandoned the site. The abandoned dam of one daredevil beaver lies across the creek only 30 feet upstream from the 120-foot drop of Bridal Veil Falls.

I do not know when these dams were built nor if they were all actively maintained at the same time, creating the same type of stair-stepped stream course present downstream from the falls. What I do know is that the same sequence of abandoned beaver dams, one after another along a creek, is present along every major stream I have walked on the eastern side of Rocky Mountain National Park. From Cony Creek at the southern end of the park to the North Fork Big Thompson River and the Cache la Poudre River at the northern end of the park, the riverscapes are haunted by the ghosts of beavers past.

Many of these now-abandoned dams were built in valleys too narrow and steep to support a beaver meadow. While maintained, each dam would have created a small pond and a miniature wet meadow as the pond filled. Only the wider portions of each stream course could support a fully developed beaver meadow. These wider portions mostly coincide with the segment of valley just upstream from the Pleistocene glacial terminal moraine.

Even if not every dam was occupied simultaneously, the hundreds of dams present on the eastern side of the park represent a lot of beavers. I try to imagine what this landscape looked like before the first people of European descent came here to trap beavers in the early 1800s. All of the large parks that are now covered mostly in grasses with a few shrubs and isolated trees—Estes Park, Moraine Park, Hollowell Park, Beaver Meadows, Horseshoe Park—would have looked more like the beaver meadow at Wild Basin today. Each beaver meadow would have had numerous ponds stepped along the valley, with willow thickets and multiple channels branching and rejoining across the valley bottom. The steeper, narrower valleys up- and downstream from each meadow would have held far more standing water because of the small pond upstream from each beaver dam. The beaver meadows would have functioned like thick, wet sponges, absorbing water in ponds and belowground sand and gravel layers during snowmelt runoff and then gradually releasing the stored water during autumn and winter. The transformation of river valleys that occurred with the disappearance of beavers I call the great drying.

This image of a vanished landscape is intriguing because a beaver pond is not just a pool of standing water. By just about any measure, beaver ponds and beaver meadows are ecological hot spots. Compared to other, equivalently sized areas on the landscape, beaver meadows have a greater diversity of plants, from microscopic algae to trees. Studies across the northern hemisphere show that beaver meadows are home to more species of insects and other invertebrates such as mussels or crayfish, as well as fish, amphibians, and reptiles, especially turtles. Waterfowl and other birds as varied as kingfishers, herons, and songbirds use the ponds and willow thickets as habitat. And mammals from mink to moose spend at least some of their time in the wet meadows. Even the bacteria are more diverse in beaver meadows. Diversity means that a greater number of distinct species of that particular type of organism are present, including species that are only present because of the wetlands created by beaver dams and canals. I love exploring the large beaver meadow on North St. Vrain Creek because I find animals that are uncommon elsewhere in Rocky Mountain National Park, from snipe to warblers and wood frogs to moose.

The slowly moving water in beaver ponds and across the floodplains of beaver meadows also encourages deposition of sediment. Trapped along with the sediment is fine-grained organic matter—the leaves, needles, and twigs shed by the vegetation along each stream. This material is rich in nitrogen and carbon that are needed by living organisms. If the organic matter keeps moving down a swiftly flowing mountain creek, there is little opportunity for microbes or aquatic insects to begin to ingest the dead plant material and absorb the nutrients into living tissues. But if the organic matter is trapped even for a few hours, let alone weeks or months, in a beaver pond, then stream organisms can begin to ingest the organic matter and take up the nutrients. Careful measurements by other scientists in beaver ponds across North America have shown greater amounts and availability of nitrogen, carbon, phosphorus, potassium, calcium, magnesium, iron, and other elements vital to living organisms in the ponds than in other portions of the stream environment.

Beaver meadows are the food larders of river ecosystems. The dams trap plant parts shed by upland and riverside forests. The plant parts are buried in accumulating, saturated muck. As I am reminded every time a misstep in a beaver meadow releases the stench of rotten eggs, the lack of oxygen down in the muck limits the presence of microbes that can release nutrients such as carbon and nitrogen in gaseous form back to the atmosphere. The nutrients are stored in the muck of beaver meadows for periods that can extend to thousands of years. But enough of the nutrients remain biologically available at the surface to support the abundance and diversity of plants and animals present in an active beaver meadow.

Environmental Fairy Tales and Complicated Realities

I return downstream along Cow Creek to the main beaver meadow. The remnant meadow is so narrow and the willows so sparse that I can see the beaver dams from the trail and keep my feet dry by mapping from a distance. Even though I appreciate the easy walking in such abandoned beaver meadows, I keep returning to the tangled, wet thickets of the active beaver meadow along North St. Vrain Creek because they are so full of surprises. Sometimes these are dangerous surprises. One year in June I inadvertently got far too close to a moose cow and calf because I did not see or hear them until they suddenly appeared among the willows. The fur on the cow’s hackles went up and I retreated as fast as I could while wearing chest waders and stumbling through beaver holes and dense willow stems. Sometimes the surprise is one of delight, as when I came on a pair of otters chattering like birds along the edge of the creek, or of wonder at something as intricate as the chewed ends of sedges that a caddisfly larva shaped into a structure resembling a tiny pagoda.

Now, as I map the locations of the old beaver dams along Cow Creek, I miss these surprises. The meadow still has a narrow band of willows along the creek and I see the lemon-yellow flash of a male warbler moving among the green leaves. But the valley bottom lacks the complexity created by numerous old beaver dams gradually being filled with sediment and marsh plants.

Where have all the beavers gone? Long time passing. Ecologists estimate that, before Europeans entered North America, somewhere between 60 and 400 million beavers occupied the continent from the Alaskan tundra to northern Mexico. Beavers had once been equally abundant in Eurasia, but trapping the animals for their fur has a much longer history on that continent. Once Europeans reached the New World, they began to trap North American beavers with tremendous enthusiasm, continuing in most regions until the species was nearly extinct.

Trappers did not reach the area around Rocky Mountain National Park until after the Lewis and Clark expedition of 1804–1806. Much of the trapping occurred during the 1820s and 1830s. Ceran St. Vrain, of St. Vrain Creek, exemplifies this history. A trapper of French ancestry, he came to Colorado from Missouri in 1824 and worked with Charles and William Bent. As part of their trapping and trading enterprises, St. Vrain and the Bent brothers established Fort St. Vrain as a fur trading center at the confluence of St. Vrain Creek and the South Platte River in 1837. They abandoned the fort by 1844 as the supply of beaver furs dwindled. John Charles Frémont described the many abandoned beaver lodges and dams that he saw during his 1842 explorations of the area, as well as the lack of evidence of active beaver colonies. Fur trappers needed only twenty years to largely wipe out beavers in the Southern Rockies.

In the active beaver meadow of North St. Vrain Creek: a moose cow and calf crossing the creek; a wood frog in an old beaver pond beside the creek (opposite page top); a caddisfly larva’s “house” in the beaver pond (opposite page bottom).

Beaver populations did subsequently recover slightly after trapping largely ceased. Ecologists estimate that 6 to 12 million beavers now inhabit North America and periodic surveys of beaver populations in Rocky Mountain National Park indicate that the animals recovered between the 1880s and 1950s.

Something happened after that. The something is a complicated story that ecologists are still trying to unravel, but which appears to have some similarities with the old nursery rhyme about Humpty Dumpty, in which “all the king’s horses and all the king’s men couldn’t put Humpty together again.”

Current understanding of the story goes this way: top predators, including wolves, grizzly bears, and wolverines, were hunted to extinction in Colorado by about 1920. Elk were also largely eradicated within Rocky Mountain National Park as a result of excessive hunting. Although residents of the area enjoyed a landscape without predators, they missed the presence of elk. Elk were reintroduced to the area from Montana in 1913, and found it good. Why wouldn’t the elk enjoy a place where hunting was no longer allowed, predators were largely gone, and park personnel placed salt blocks beside roads to attract elk, deer, and bighorn sheep for easy viewing by visitors? As noted earlier, moose were also introduced into the region around Rocky Mountain National Park and quickly made their way into the park and increased in number.

As signs of overgrazing became increasingly marked by the late 1930s, park naturalists realized that elk and deer populations were exceeding the carrying capacity of the environment. There followed decades of attempts to limit populations in the highly visible environment of a national park. In 1941, a special hunting season along the park’s eastern boundary did not result in enough killing, so in 1944 park rangers shot about 300 elk. Live-trapping and transplanting were tried subsequently, along with contraceptives, tall fences to keep elk and deer away from some of the creek-side forests, and more episodes of selective shooting.

Meanwhile, the beaver meadows disappeared. Elk like the succulent plants that grow along streams and, just like domestic cattle or sheep that are left to their own devices, elk that are not kept moving by the presence of predators will spend a large amount of their time along the streams. A herd of 500- to 700-pound elk is very effective at competing with beavers for their mutually preferred food of willows and aspens. Beaver numbers dropped steadily as elk numbers increased during the second half of the twentieth century.

Having learned this history, I knew that I had unconsciously developed a beaver-centric perspective when I was working in the beaver meadow along North St. Vrain Creek one day and came upon the first elk I had ever seen there. My immediate thought was, “The enemy!”

The story of unnaturally large elk herds displacing beavers is not unique to Rocky Mountain National Park. A similar progression through time has been observed at other national parks in the continental United States where predators were hunted to extinction. At some of these national parks, however, the predators have been reintroduced or have reintroduced themselves by migrating in from Canada. Yellowstone National Park is the prime example of this phenomenon and in some ways represents the inverse of the story just described for Rocky Mountain National Park.

Wolves were hunted to extinction in Yellowstone by 1926, elk subsequently flourished, and beaver numbers dropped. With the reintroduction of wolves to Yellowstone in 1995, an interesting recovery began. Scientists excitedly documented how hunting wolf packs kept elk herds on the move, culling the weak or aging elk just as wolf advocates had been describing for many years, improving the health of the elk population and helping to limit the number of animals to a level that the environment could sustainably support. The unexpected bonus appeared along valley bottoms where reduced grazing by elk allowed the renewal of plant communities not seen in decades, including dense willow thickets and stands of aspens. Soon the beavers followed, building dams that caused snowmelt floods to spread across the valley bottoms rather than shooting downstream wholly within a single channel. The higher water levels associated with the beaver meadows kept the water-hungry willows and aspen growing steadily, assuring the supply of beaver food and the continuance of beaver colonies and meadows.

This is a wonderful story. The predators return and the ecosystem comes back into balance, with a place for beavers and all of the abundance and diversity that they support by building dams. Scientific understanding of ecological interactions leads to public awareness of the importance of predators and enlightened management returns more natural riverscapes to the national parks. Except … except it’s not that simple.

Alternate Rhythms

When beavers abandon a site, their dams fall into disrepair. Decrepit dams no longer effectively block the stream channel and seasonal floods are less likely to spread across the valley bottom. Swift, high flows contained within a single channel are more erosive, widening and deepening the channel and increasing the likelihood that the water will remain within the channel rather than overtopping the banks. Beaver ponds on and beside the main channel drain and the secondary channels go dry. The water table across the valley bottom drops. Wetland plants disappear and the soil dries.

New pioneers move in. Among these are burrowing rodents such as voles and pocket gophers. These rodents eat plants and the ectomycorrhizal fungi that live in soil within the roots of spruce and fir. The trees cannot survive without the fungi, which help the trees to absorb nutrients from the surrounding soil. Burrowing rodents spread the spores of the fungi by excreting the spores in their feces. As the fungi colonize the newly dry soils, spruce and fir can also colonize the site. The upland plant community gradually moves down into the now relatively dry valley bottom, creating what is sometimes called an elk grassland.

Now the soil is on the dry side for willows and aspen and the main stream has cut down below the level of the surrounding grassland. Even if someone dropped off a pair of happily mated beavers and invited them to be fruitful and multiply, the beavers would have a tough time of it. There are not enough of their preferred woody plants to feed the beavers or to provide dam-building materials. The situation is not impossible. Beavers are resourceful. They will build dams out of mud and cobbles and they will eat grasses and conifers, but deciduous woody plants are the most important part of their diet, as demonstrated by an experiment just north of Rocky Mountain National Park.

An abandoned beaver meadow with overgrazed, isolated clumps of willows and a drier valley bottom along which only a single channel flows. This is Upper Beaver Meadows in the national park.

In 1999, the US Forest Service reintroduced beavers to a site in the Medicine Bow National Forest in southern Wyoming that had once supported them. The meadow had subsequently been heavily grazed by cattle and had no willows, so the Forest Service dropped off cut aspen trees for the beavers to eat. After three years, the supplemental food supply stopped and the beavers vanished.

The Wyoming site, along with some of the valley bottoms in Yellowstone and in Rocky Mountain National Park, had entered what ecologists call an alternative stable state. This is a little bit like a steeply descending trail that forks to reach two different valleys. Follow one fork far enough and you come to a valley that seems like a perfect camping spot, not least because to return uphill and reach the other valley would require a significant amount of time and energy. Either valley provides a nice destination, but you need to invest enough energy to climb back out of one valley to reach the other.

Alternative stable states describe two possible scenarios for an ecosystem. Either alternative can remain stable for a long period of time, but some major change is required to shift the ecosystem from one alternative to the other. In a wide valley upstream from a terminal moraine, the presence of beavers and their dams maintains a wet and diverse valley bottom with areas of swiftly flowing water, recently ponded water, active dams, abandoned dams overgrown with willows, and old ponds gradually filling with sediment. As long as the beavers remain and the valley meets some minimum size that allows new trees to replace those gnawed down by the beavers, this configuration can persist for hundreds to thousands of years—a very stable state. But if something causes the beavers to leave the site, the wet meadow can transition to a dry elk grassland that is gradually invaded by upland vegetation, and this new stable state can also persist for a very long time. Some large input of energy or a dramatic change is required to cause the transition from one stable state to another, and returning to a previously existing stable state is not necessarily straightforward. This seems to be the case in Yellowstone, where some valley segments are returning to beaver meadows, but others are not.

Efforts to limit elk populations in Rocky Mountain National Park continue. Some of the former beaver meadows, such as Upper Beaver Meadows, have tall fences to exclude elk and deer. The difference between the groves of willow and aspen within the fenced area and the grasses and small shrubs outside the fence is striking. The fences are part of long-term riparian restoration being undertaken by the park service. The idea is to allow large enough riparian forests to grow back to support a beaver colony. Once the beavers are able to build dams and feed themselves, they will create beaver meadows that will be self-sustaining. This strategy will take decades to implement, but it should work if elk grazing in the meadows can be kept to a level that allows the willows, aspen, and beavers to survive.

I return past Bridal Veil Falls to the shrunken beaver meadow along Cow Creek. I imagine the day when Moraine Park and Upper Beaver Meadows will once again be places where only the determined bird watcher or fisherman ventures into a maze of sticky-bottomed ponds and small holes hidden among the tall sedges. In the shaded ground along the valley bottom of Cow Creek, the leaves of wintergreen promise that new growth is returning and the meadow will soon be vibrant with flowers and insects. I think about the choices we can make to renew the beaver meadow here and wonder if someday I’ll hear the slap of a beaver tail on a pond at dusk along this creek. I hope so.