Enhancing Riparian Habitat Restoration with Coarse Woody Material

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by Corie Schott & Geoff Elliott, Grand Environmental Services

Reduced river flows in the Rocky Mountain West are a challenge to us all. Altered hydrographs, shrinking and disconnected riparian corridors, unstable river banks, and lack of coarse woody material (CWM) are all part of a new western hydrogeomorphic reality which forces us to rethink our riverbank designs, looking for creative, cost-effective methods to restore riparian functions.

Coincidentally, the recent pine beetle epidemic generates an abundance of inexpensive CWM in forested parts of the West, including Grand County, Colorado. We here offer some field perspectives on enhancing riverbank restoration with different size classes of CWM, focusing on recent work at the Colorado River in Pioneer Park-Hot Sulphur Springs State Wildlife Area, including a pilot project in 2003 and larger scale work this fall.

In our riverbank restoration designs, we use three size classes of CWM — large, medium, and fine — to stabilize the riverbank and to expand a higher functioning riparian habitat. This technique mimics the accumulation of CMW in natural riparian systems and foster the life cycle components and food-chain support necessary for a large number of species. Our aim is to grow more stable riparian systems with greater integrity that also produce measurable functional values including:bank stabilization and sediment control;enhanced flood-flow attenuation, groundwater recharge and conjunctive flow; and food-chain support and wildlife habitat (see Figure 1 and Table 1).

 

Table 1: Coarse Woody Material (CMW) Size and Functional Values

CWM Size and Description Primary Design Function Secondary Design Functions Riparian Functional Values (Metrics) Target Wildlife Species at Pioneer Park
Coarse — Logs with and without root wads Bank structure (sides)
Hydraulic structure (reduce near-bank stress).
Aquatic refugia
Food chain support
CWM capture
Bank stabilization
Flood-flow attenuation
Food chain support
Wildlife habitat
Fish such as brown trout, brook trout, rainbow trout
Small mammals such as beaver, river otter, mink, muskrat, water shrew, vole
Birds such as bald eagle, Northern harrier, osprey, kingfisher, great blue heron
Medium — smaller logs, branches, shrubs Bank structure (top)
Reduce flood velocities
Protect vegetation
CWM capture
Microhabitat
Food chain support
Bank stabilization
Flood-flow attenuation
Groundwater recharge
Food chain support
Wildlife habitat
Aquatic insects such as dragon flies, caddis flies, nymphs
Reptiles such as the garter snake
Amphibians such as boreal toad, N. leopard frog, tiger salamander
Small mammals
Migratory songbirds such as Wilson’s warbler, flycatchers
Fine — sticks, wood chips, straw Bank structure (internal)
Soil hydraulics (wicking)
Vegetation support
Microhabitat
Food chain support
Groundwater recharge
Conjunctive flow
Food chain support
Wildlife habitat
Aquatic insects
Reptiles
Amphibians
Small mammals
 Figure 1.

Large CWM — Large diameter logs, with and without root wads, offer great riverbank structure in areas like the Upper Colorado River where logs are only rarely part of the river system due to harvest, clearing, and lack of flushing flows. Large CWM can be difficult to work with, however, and requires experienced heavy equipment operators. Wind-throw cottonwoods are definitely worth the effort, as they fit better into the riverine ecosystem, offer greater coarseness, variability, and heft. Beetle-kill lodgepole pine trees are generally easier to work and there are literally mountains of material available, but they look more regular, which is negative in terms of aesthetics. Both can be had for the cost of transport, which is not necessarily cheap. We work with Grand County Department of Natural Resources.

Medium CWM — We stack smaller logs, branches, and large wood chips to mimic debris dams, log jams, and beaver dams in natural systems while enhancing internal bank structure. These materials are very inexpensive (mostly free right now), and much easier to find and use than larger CWM. For instance, the material can be moved by a front-end loader or by hand.

Fine CWM — Materials such as sticks, woodchips, and straw are placed under, around, and over large and medium sized CWM and mixed with river sediment to fill the structures and create a basis for soil development and vegetative growth. These materials are very inexpensive to acquire and easy to use with a front-end loader or even wheel barrow, and we have experienced a high success rate.

We measure functional values associated with CWM by metrics accepted by the US Army Corps of Engineers.

Bank Stabilization/Sediment Control — All three size classes of CWM can be used toincrease bank stability. At Pioneer Park, CWM collects sediments and wetland plant seeds and leads to riverbank growth with little to no maintenance. Large-diameter downed trees offer integrated riverbank structure, stability and durability while the created coarse edge reduces near-bank sheer stress, captures and holds smaller CWM, and reduces sediment passage. Medium CWM enhances internal structure and stability of the riverbank. The added roughness in turn captures and retains other size classes of CWM, and further mimics natural riparian systems. Fine CWM mixed with riverbank silt provides a base for wetland vegetation to grow and develop massive root structure, which increases stability.

We measure bank stabilization and sediment control by metrics associated with topography, vegetation, and hydrology. Stable banks are usually rough and irregular, well-vegetated, possess structural integrity, resist erosion, and trap and retain new CWM (e.g., drift lines and debris dams).

Flood-Flow Attenuation — The use of both medium and fine CWM increases the riparian ecosystem’s ability to improve ground water recharge and conjunctive flows. Medium CWM improves the riverbank’s ability to receive overbank flows from a stream or river and store that water for some period of time. The added roughness on new floodplain surface at Pioneer Park improves flood flow attenuation and in turn groundwater recharge. Fine CWM improves the riverbank’s ability to receive and absorb flows from a stream or river and store that water. This results in groundwater recharge and conjunctive flow through absorbing, channeling, holding, and wicking of water. In low-flow conditions, fine-grained CWM helps wick water from the riverine system into the bank.

We measure flood-flow attenuation by metrics associated with microtopography, vegetation and hydrology. Habitats with high flood-flow attenuation have hummocky microtopography with CWM. Banks are densely vegetated and have a relationship to bankfull flows, overbank flooding, and side channels.

Food-Chain Support — All size classes of CWM offer the substrate for food-chain support. CWM enhances the wetland’s ability to provide an organic foundation for the start of the aquatic food chain by providing surfaces for microbial activity used by vegetation and aquatic insects, which in turn provides food-chain support for other species. The use of different size classes of CWM also creates pockets of collected water where insect larvae can develop even during higher flow events.

We measure food-chain support by metrics associated with vegetation, substrate, and hydrology. Riparian habitats offering high food-chain support generally have well-vegetated stream banks and evidence of organic growth on log surfaces including captured debris/litter. In addition either groundwater discharge, or evidence of seasonal inundation must be evident with drainage patterns.

Wildlife Habitat — The use of all three size classes of CWM in riverbank restoration can lead to enhanced wildlife habitat value through refugia and food-chain support. CWM enhances the riverbank’s ability to provide security/cover for insects, amphibians, reptiles, juvenile fish and small mammals, which in turn provides food-chain support for other species. Trout, river otter, beaver, mink, and muskrat can benefit from the use of CWM in riverbank restoration as well as various migrating songbirds and birds of prey including bald eagle, northern harrier and osprey. CWM also results in microhabitat to foster riparian vegetation, leading to better vegetated stream banks and habitat connectivity.

We measure Wildlife Habitat value by metrics associated with plant species diversity, community/structural diversity and presence of snag trees. We document species observations, use of travel corridors, evidence of tracks, scats and other sign, and presence of beaver dam complexes for evaluating level of activity. Colorado Division of Wildlife (CDOW) maps also support our restoration design and monitoring. For example, the Division’s data from Grand County in 2003 tells us that both the federally listed bald eagle and the state listed river otter occurs along the Colorado River at Pioneer Park. Therefore, we employ restoration techniques that 1) enhance their prey base by supporting a healthy fishery and providing suitable small mammal habitat, and 2) reconnect the riparian corridor to facilitate migration and dispersal.

We hope other restoration practitioners will consider increasing use of three size classes of CWM in riparian habitat restoration. We have experienced successful results in our design at the community project on the Colorado River at Pioneer Park. We encourage the use of this cost-effective restoration technique to mimic what would have been natural riverbank accumulations during historically higher flows, and to improve riparian functional values that are measurable.

This work is sponsored in part by the Town of Hot Sulphur Springs and Great Outdoors Colorado, and accomplished in collaboration with the US Army Corps of Engineers, Colorado Division of Wildlife, Grand County Department of Natural Resources, and Natural Resources Conservation Service.