Watershed Restoration

by Tao Orion

This excerpt is adapted from Tao Orion’s book Beyond the War on Invasive Species: A Permaculture Approach to Ecosystem Restoration (Chelsea Green, 2015) and is printed with permission from the publisher.


Aquatic ecosystems have been largely mismanaged, and the growth of invasive species demonstrates the lack of appropriate interaction with these valuable natural resources and the ecological services they provide. Invasive species proliferate in polluted, nutrient-enhanced, or otherwise compromised waterways. They also exist in ecosystems that appear somewhat intact, though historic analysis can tell a different story and helps to explain the presence of some of the more vexing invasive species, such as Japanese knotweed and giant reed.

Beavers shaped the watersheds of North America over the course of the millions of years they have been present on the continent. These busy ecosystem engineers altered stream flow and shaped aquatic ecosystems from high mountain streams to alluvial floodplains in the process of building their homes. Beaver dams are unique structures—semipermeable, strong enough to walk on, and often inhabited for generations. They build these structures from trees, sticks, and mud, stacking them together so that the bulk of the stream flow is held behind the structures, but also with spillways and passageways so that water can move through without damaging their homes. The inside remains dry, though the entrance is submerged. These physical characteristics slow down floodwaters and capture sediments.

Prior to the arrival of European colonists, beavers inhabited creeks and wetlands from the arctic tundra of Alaska and northern Canada to the deserts of northern Mexico. Beaver trapping was one of the first industries to take hold on the newly “discovered” American continent—in many cases, fur trappers were the first people of European descent to explore the land west of the Mississippi and north of the Hudson Rivers. Their hunting and trapping, which fed the demand for beaver coats and felt hats in Europe, led to the near extirpation of the species by 1900.

Hunting and trapping of the beaver in seventeenth- through twentieth-century North America had extraordinary effects on the ecology of the continent, from the physical structure of riparian areas and floodplains to the species composition and richness of aquatic ecosystems.

It is estimated that as many as two hundred million beavers once lived in the continental United States, their dams making meadows out of forests, their wetlands slowly capturing silt. The result of the beaver’s engineering was a remarkably uniform buildup of organic material in the valleys, a checkerboard of meadows through the woodlands, and a great deal of edge, that fruitful zone where natural communities meet. Beavers are a keystone species, for where beavers build dams, the wetlands spread out behind them, providing home and food for dozens of species, from migrating ducks to moose, from fish to frogs to great blue herons.1

Where beaver populations are undisturbed, they have been observed to construct around ten (but up to seventy-four!) dams per kilometer of stream.2 Where water once moved slowly and placidly through a series of beaver-constructed pools as it made its way from ridge top to river mouth, in the absence of these permeable dams, water moves faster through the channel, without as much time to soak in for use by surrounding vegetation. Beaver dams also influence off-channel hydrology by making the environment more moist. A beaver dam in Minnesota measuring 6 feet by 250 feet influenced the hydrology within half a mile of its location. Lack of beavers means fewer semipermeable dams, less water volume soaking into the surrounding land, and, therefore, reduced aquifer recharge.

It is hard to imagine a time when streams and rivers pulsed and changed with the natural patterns of flood and drought, their courses shifting and meandering in braided networks of side channels, rather than confined to a singular path. In the absence of beavers, and concomitant with the construction of impermeable concrete dams and other significant alterations in hydrology, streams and rivers are now constrained to a specific space, where their flows continually erode sediments, unmitigated by side channels, back eddies, or flow-reducing structures like beaver dams. Today, beavers are considered nuisance pests in many areas because their dam building and resultant flooding damages property.

A lack of beaver dams or similar structures in a waterway does not mean there is less water moving through the channel, but that the same amount of water moves downhill with greater force, and the sediment that the water carries moves quickly to the ocean. Rapidly-moving water carrying sediment is often turbid, as the sediment is not collected by any flow-decreasing structure. The increased velocity of water also contributes to the channelization and scouring of stream banks, many of which today are highly aggraded with steep channels. Beaver-structured creeks and their clear, slower moving waters yield perfect spawning grounds for salmon and other anadromous fish, like trout, char, grayling, and whitefish, which require clear, cold, relatively slow-moving water with gravels of a certain size to lay their eggs. At least eighty species of fish, including many that are critically endangered like sculpins, sticklebacks, and suckers, have been found in beaver ponds, where they benefit from the slow-moving water; they don’t have to expend so much energy fighting against the currents in search of food.3

Beavers were also instrumental in creating some of the best agricultural soils on the continent—floodplain soils. Creek and riverside farms may owe their fine fertile soil to beavers, since water slowing down and spreading out behind a beaver dam widens the lens of water and the sediment it carries to beyond the stream channel. Over time, the depositional activity results in a series of flat terraces moving down the stream channel in a stair step pattern. One study found that beaver dams retain from 35 to 6,500 cubic meters (m³) of sediment behind their structures.4 If this material were used to fill 55-gallon drums, 32,500 would be required to capture the sediment behind the largest beaver dams. Over time, streams turn to wetlands, and wetlands to meadows, meadows to prairies, and eventually, a long time after the beavers are gone, to farms.

Beaver dams are considered ecologically beneficial because they conserve water, nutrients, and sediment and especially because the dams hold water higher up in a watershed before it makes its inevitable way downstream. The secondary effects of these ecological services promote the ability of other species to thrive, including fish, insects, birds, mammals, and a wide variety of riparian vegetation that depend on the riparian channel profile created by busy beavers. The total sum of ecosystem services yielded by beavers is difficult to quantify, but one thing is certain—the unprecedented reduction in their populations through historic hunting and trapping has rippled throughout the riparian ecosystems where they were once found and contributes to the modern proliferation of riparian invasive species.

Interestingly, many of the same traits are assigned to the invasive giant reed but are viewed as negative ecological effects. Giant reed is targeted as a noxious invasive weed in California waterways because it increases local flooding by spreading water laterally beyond a channel, increases sediment retention, and changes the direction and force of in-stream currents and therefore decreases channel velocity.5

In the absence of an animal whose engineering feats increased sediment retention, spread water beyond the channel, and decreased water flow velocity, there is a plant growing that does many of the same things. As Parmenides mentioned nearly two thousand years ago, nature abhors a vacuum. Robust stands of giant reed do preclude the growth of native riparian vegetation. They also don’t provide habitat for the diversity of animals that native species do, but given the alteration of stream structure engendered by the loss of beavers and other related changes in land use and hydrology, there is no indication that native riparian vegetation, including flood-adapted willow and cottonwood, would survive in the altered conditions.

Short of widespread beaver reintroduction, which should also be a serious consideration for restoration of North American waterways, making use of the physical and structural characteristics of invasive riparian species like giant reed and Japanese knotweed is a viable option that over time, could produce similar ecological results. Giant reed is one of the most prolifically growing plants on the planet. It can grow up to 7 centimeters per day and produces 60 tons of biomass per acre. Its canes are similar in strength to bamboo and, when cut, would make suitable material for building woven weirs across stream channels. Riparian restoration often calls for the placement of gabions—semipermeable rock-filled cages behind which water and sediment can pool—in streams. Gabions are extremely effective for enhancing stream structure and flow, but they are energy and resource intensive. Rocks are often trucked in to the site at great expense, and eventually the metal cages that hold them will rust away, leaving huge piles of rocks in the reemerging stream corridor. A more ecologically based approach would make use of giant reed’s abundant biomass by following the permaculture principle of using biological resources.

Sturdy anchor canes can be placed directly into the streambeds to provide the initial structure, much like the ribs of a basket. Additional canes can be woven between these, creating a semipermeable, long-lasting (but still decomposable) structure that encourages the deposition of sediment and pooling of water. These structures can be made similar in shape and size to beaver dams, around 6 feet tall and 5 feet deep, and as wide as the span across the stream channel. Once the woven structure is made, the prolific leafy biomass of the reed can be layered into the basket. Over time, the material will break down, and the rhizomatous reeds will resprout. At this point, the canes can be cut again, adding further material to the water retention structure. If these natural gabions were placed at the same concentration as historic beaver dams (10–70 per kilometer), not only would the population of giant reed decline, but the stream would also start to change in character and ecological quality.

By consistently pruning back its photosynthetic leaf surface, over time, the vigor of the reed’s rhizome would decrease. The decomposing reed mulch in the gabion would turn to soil over time and could eventually be planted with willows and cottonwoods, providing shade and habitat to the riparian corridor. The gabion structures, like beaver dams, allow water to pass through, but build up sediment behind them. Eventually, the stream channel would attain a stair step gradient again, the channel would widen, and more water would stay in the watershed longer. Seasonal floodplains would form, and soil would be kept from the sea. Native riparian vegetation would flourish in the rehabilitated stream channels.

References

  1.  Alice Outwater, Water: A Natural History, (New York: Basic Books, 2008).
  2. Michael M. Pollock, Morgan Heim, and Danielle Werner, “Hyrologic and Geomorphic Effects of Beaver Dams and Their Influence on Fishes,” in American Fisheries Society Symposium, vol. 37, (2003), 213-233.
  3. Ibid.
  4. Robert J. Naiman, Carol A. Johnston, and James C. Kelley, “Alteration of North American Streams by Beaver,” BioScience (1988):753-762.
  5. David F. Spencer, Liz Colby, and Gregory R. Norris, “An Evaluation of Flooding Risks Associated with Giant Reed (Arundo donax),” Journal of Freshwater Ecology 28, no. 3 (2013): 397-409.

About the Author

Tao Orion teaches permaculture design at Oregon State University and at Aprovecho, a 40-acres nonprofit sustainable-living educational organization. She consults on holistic farm, forest, and restoration planning through Resilience Permaculture Design, LLC. Tao holds degrees in agroecology and sustainable agriculture from UC Santa Cruz and has a keen interest in integrating the disciplines of organic agriculture, sustainable land-use planning, ethnobotany, and ecosystem restoration in order to create beneficial social, economic, and ecological outcomes.

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