Purpose and Objective

The objective of the streambank stabilization study was to test the effectiveness of several treatments in stabilizing eroded streambanks within the demonstration area. Practical and cost-effective techniques included structural measures (check dams), biological treatment (revegetation), revision of land management practices (land management), and reinforcement of streambanks with new nonstructural bio-geotechnical methods (Gray and Leiser 1982).

Methods

Check Dams

Four loose rock check dams were constructed at intervals in a half-mile reach of the stream to create a series of ponds that would reduce flow velocity, channel gradient, and streambed downcutting. A local rock outcrop was blasted to obtain 2,520 cubic yards of rockfill material for the dams (USDA SCS 1985). Local soils were compacted to form watertight cores in the dams. Each upstream dam was constructed 3–4 feet higher than its downstream neighbor to form a series of steps in pond elevation, with the head of each pond submerging the toe of the next dam upstream. Since they were built at points determined by channel gradient and location of bedrock anchor points, their spacing is irregular (Figure 5-1). The distances between Check Dams #1, #2, and #3 were relatively short, indicating a steeper stream gradient; the distance between Check Dams #3 and #4 was long, indicating a much flatter gradient. The overall height of each dam was in the range of 5–10 feet as measured from the keyway foundation to the highest part of the abutments.

The dam designs were developed in accordance with SCS design standards, with some structural modifications to make them more secure (USDA SCS 1985). The dams were keyed into the banks approximately 10 feet (further than required by the standard) to ensure against washout of the abutments. The downstream aprons of the spillways were graded at approximate slopes of 10:1 (the standard design required 5:1). The flatter and longer aprons provided much more energy dissipation to protect against erosion at the downstream toes of the dams and greater stability against movement of dam materials by high flows (Figure 5-1)

Spillway cross-sections at the dams were sized to accommodate a ten-year flood flow. At Check Dams #3 and #4, where the banks were relatively low, earthen dikes were extended from the dams onto the floodplain to divert flows away from the dam abutments in the event that high flows overtopped the dams and banks. The ponds reduce the velocity of the flow, which decreases the erosional energy of the water and causes deposition of suspended sediments. The dams also immediately raise the elevation of the shallow water table out into the floodplain, increasing aquifer storage capacity and creating more desirable growing conditions for plants (see Section 13 for more details).

Revegetation

A revegetation plan was implemented to accelerate the recovery of vegetation in areas impacted by construction of the check dams and to reintroduce desired woody and herbaceous species into the plant community. A key project objective was to test the feasibility of reestablishing native species where possible. Planting took place in the study area in fall 1986 and spring 1987 (Figure 5-2). Treatment variables included several hardwood plant species (unrooted cuttings and rooted liners), two planting seasons (fall and spring), and three zones of available moisture (high, medium, and low) based on elevation above the water table (Table 5-1). Two herbaceous seed mixes were also tested (Table 5-2). Lindquist, McCalla, and Filmer (1994) provide detailed planting and monitoring results.

Woody plantings along the streambanks consisted of unrooted cuttings of locally obtained coyote willow and black cottonwood and rooted nursery stock (liners) of several hardwood species (Table 5-1). Rooted liners were planted on the top of banks; unrooted cuttings were staked into vertical bank faces and bank toes and along rock keyways.

Two herbaceous seed mixes were tested—a commercial mix of crested and intermediate wheatgrasses and alfalfa and a mix of native western wheatgrass and Great Basin wild rye (Table 5-2). The seed mixes were broadcast on top of the banks and construction areas, fertilized, raked, and in some areas mulched with straw (Figure 5-2). This presented an opportunity to test the survival rate of native species over a commercial seed mix in stabilizing denuded areas.

Monitoring data were collected in 1987, 1988, and 1994 to determine the most successful planting treatments in terms of survival and overall cost. Surviving woody plants were counted each August by species, plant form, moisture zone, and planting season. Percent cover of herbaceous vegetation grown from seed mixes was estimated visually. In 1994, field observations were made to provide qualitative assessment of the results after seven years.

 
 
 
 

Table 5-1

Evaluation of Revegetation Plantings with Hardwood Species After Two Growing Seasons

Source: Lindquist, McCalla, and Filmer, 1994, Erosion Control Demonstration Project in Red Clover Valley: Revegetation/ Streambank Stabilization Program.
N/A = Not applicable. No planting in that zone.
^a High moisture zone (wet) = 0–2 feet, medium moisture zone = 2–4 feet, low moisture zone (dry) = > 4 feet.
^bSuccess rates: E = excellent, G = good, P = poor.
^cThree liner willow species include Columbia River willow, coyote willow, and purple osier willow.

Note: All planting done in the high moisture zone.

Land Management and Controlled Access

An exclosure fence consisting of five strands of barbed wire was constructed around approximately 65 acres. The fence prevented cattle and vehicle disturbance to vegetation, soils, and check dams within the project area. The original landowner agreed not to graze the exclosure area until 1991 (five years) but voluntarily deferred grazing until 1994, when a grazing plan developed by FRCRM and the new landowner was put into effect. The plan prescribed short-duration grazing (five days) within the exclosure to increase plant vigor and stimulate new growth. Grazing was planned for late season (August) to avoid conflict with waterfowl nesting. After eight years of protection from grazing, grasses and willows were thriving along the channel and in low-lying areas of the meadow within the fenced area. In contrast, intense cattle grazing outside the exclosure removed most of the vegetation, leaving soils vulnerable to erosion.

Bio-Geotechnical Treatments

Two bio-geotechnical treatment methods were undertaken to demonstrate the effectiveness of nonstructural, less costly restoration treatments. The methods were selected based on cost, feasibility, and the availability of local materials. The first was installation of a cut-tree revetment at the north bank of Pond #1 in 1987 and 1988 (Figure 5-3). Small Jeffrey Pines were placed butt-up along the face of the bank and secured to the top of the bank with cables (Sheeter and Claire 1981). The trees were expected to reduce water velocity, protect the streambank from erosion, and filter and deposit sediment, thereby allowing vegetation to establish. The second method was reinforcement of an eroding bank with willow matting (locally collected plant stock tied together to form a mat structure). The mat was then

staked down with heavy wire along the south bank of Pond #1 in 1991 to arrest erosion where strong currents impinged during high flows.

Key Findings

Check Dams

• The check dams were effective in slowing the water velocity through the project site during high flows, thereby reducing the erosive forces on the stream bottom and banks. This gave the revegetation measures a better chance of success and reduced local recruitment of sediment to the stream. The upper dam required maintenance after the seventy-five-year flood event in 1986, but otherwise survived several subsequent twenty-five-year events with minimal effect.
• Check Dam #2 appeared to cause currents to form on the north bank upstream of the dam; the currents eroded the south bank of Pond #1, which damaged the willow matting. Some care is required in the layout and design of check dams to ensure that jet flow downstream of the spillways does not impinge on erodible banks and is consistent with geomorphologic features of the system.
• The check dams were also effective in raising the elevation of the stream, which over time led to seasonal recharge of the shallow water table. This had a very positive effect on establishment and growth of vegetation along the channel and far out on the floodplain and, along with the exclosure fence, was largely responsible for the quick site recovery and dramatic increase in waterfowl use.
• Check Dam #4, heightened by a beaver addition, caused overland flow during high flows, which, upon reentry to the main channel below Check Dam #1, caused bank failure and headcutting of an old shallow channel. Again, careful attention to design of the check dams is required to address this potential problem. Where overland flow is likely, reentry points need to be identified and armored if necessary to prevent bank erosion and/or headcutting.

• Many plants established reasonably well, in spite of persistent drought. Overall survival of woody species was 44% during the first year (1987) and 23% during the second year (1988). Fall-planted unrooted cuttings, however, had consistently higher survival rates than rooted liners for most combinations of treatments—33% survival for stakes and only 17% for liners after the second growing season (Figure 5-4). Spring plantings showed similar survival rates for both stakes and liners after two growing seasons; based on a 1994 monitoring survey, survival rates for unrooted stakes were higher than rooted liners for all treatments. Continued drought severely desiccated rooted liners. Plantings in the highest moisture zone (0–2 feet from the water table or surface water) were most successfully established regardless of species or plant type (Table 5-1, Figures 5-5 and 5-6).
• Overall, locally collected unrooted willows planted in the fall in the high-moisture zone were most successfully established, even in rocky sites along the dam keyways. Planting unrooted stakes provides several advantages over rooted materials, including lower cost to obtain materials because they are collected locally, less time required to install per plant, less stress of acclimation because they are already adapted to local climatic conditions, broader moisture tolerance due to their 18-24 inch length, lack of an initial root structure to support, and ability to survive on sloped and flat areas. Willow stakes can survive up to three years on carbohydrate reserves alone and therefore tend to be more drought-resistant than rooted materials.

• Results after the first year of planting were not necessarily indicative of what plant type and season showed the most promise. Overall, results between treatments (i.e., fall vs. spring planting and liners vs. stakes) were not appreciably different after the first year, but these numbers changed dramatically after the second and third growing seasons. Therefore, monitoring for at least three years is recommended to compare treatments more accurately.
• For best results, unrooted stakes should be collected and used while dormant and should be cut as long as possible (24–36 inches) based on site conditions. This improves access to deeper water. In rocky areas, stake plantings should be preceded with a pilot hole to avoid damage to the bark while planting. Willow stakes should be soaked for 48 hours before use to dissolve rooting inhibitor hormones.
• Drought conditions and climatic variability were the overriding factors affecting plant survival, especially in the first three years after planting. Soil moisture was a critical factor. Better survival rates were achieved in the high- and medium-moisture zones (Table 5-1, Figures 5-5 and 5-6).
• Of the rooted liner species, quaking aspen had the best results compared to mountain alder, black cottonwood, and douglas spirea, which did poorly (Table 5-1). Rooted liners are sensitive to weather variations and require planting in high-moisture zones; alternatively, irrigation may be required to establish the plants. They were also more labor-intensive to plant and more costly overall. Rooted liners used in this study were obtained from a nursery in Oregon and were therefore not acclimated to site conditions. This could partially account for their poor performance.
• Mortality rates were higher than expected for all species in this study due to, among other factors, prolonged drought during the study period, planting before plants were dormant in some cases, extreme climate variability, streambank sloughing, poor condition of some liner plant materials, beaver activity, and planting species in low-moisture zones where they would be unlikely to survive. Additional information on natural regeneration of vegetation is presented in Section 8.
• Initial seeding results were more successful with the commercial seed than the native plant mix. Cover increased to 50% the first year with the commercial mix, providing more immediate erosion protection to barren banks and berms. The native seed mix was slow to produce results, but after three years, Great Basin wild rye was well established on a dryer site away from the channel. Western wheatgrass establishment was poor even after three growing seasons (Table 5-2). The lag time for establishing native species should be taken into consideration when planning future projects. Planting of native species is important to establish sustainable long-term grass coverage. It is also important to obtain locally harvested seed materials to promote more successful germination and establishment.
• Planting of woody species on raw over-steepened or vertical streambanks should be deferred at least two years following a project due to sloughing of the banks. Mechanically sloping back streambanks would reduce sloughing and accelerate revegetation and stabilization, but it is costly and in many cases not feasible. Natural vegetation regrowth was extensive after the third growing season, once the channel began to stabilize and the water table was recharged.
• Impact of cattle, beaver, and humans must be controlled until new plants pass at least three growing seasons to provide the best conditions for successful establishment. In 1994, beaver impact was severe in some locations (mature willows were decimated by the beaver population). Tours of the project in vehicles and on foot also caused some plant mortality and must therefore be controlled.
• A survey of plant survival conducted in 1994 showed that natural regrowth of vegetation was filling the gaps between surviving planted vegetation. Point bars and the toe along the streambank were well vegetated. Willows planted in keyways and along earthen berms were surviving and firmly established, whereas most plants within the channel were dislodged due to bank sloughing. Of the rooted liners, only a few aspen survived. The commercial seed mix was well established and provided good protection for soils disturbed in construction; alfalfa was frequently used by deer and small mammals for food and cover. Of the native grass seed mix, only the Great Basin wild rye survived and was thriving in pockets of dry sandy soils away from the channel.

• The exclosure fencing kept cattle and motorized vehicles out of the demonstration area for eight years. Though infrequent, trespass by vehicles and even extensive foot traffic caused mortality of some vegetation planted in the exclosure. The reduction of disturbance helped establish vegetation and stabilize streambanks. The flourishing hardwoods (willows) and herbaceous vegetation (grasses and sedges) provided protection to soils and habitat for waterfowl and wildlife and produced a stark contrast to barren ground outside the exclosure.
• A grazing management plan reintroduced controlled cattle grazing in the exclosure in 1994. Unfortunately, cattle remained in the exclosure for several weeks, denuding most vegetation and trampling streambanks. In response, the FRCRM Grazing TAC developed draft guidelines for grazing management at future projects. Identification of improved land management practices is a key component of successful watershed restoration and should be built into projects in the initial stages of planning. Site visits in 1995 confirmed that significant vegetation regrowth did occur in the exclosure, but mechanical damage to streambanks from cattle trampling was still apparent and slower to heal.

• Due to reduction in grazing at the downstream control site, remarkable improvement was observed after 1989. The creek showed marked signs of healing without any other measures being undertaken. At this location, the channel narrowed and began to meander, riparian vegetation reestablished along the banks, fish became established, and bank stability improved. In some instances, changes in land management alone can turn around the degradation process, albeit at a slower rate.

• The cut-tree revetment was effective in promoting stabilization of the north bank of Pond #1. The revetment filtered sediment that deposited at the base of the streambank. This provided substrate for establishment of vegetation, which in turn enhanced stabilization. This area is continuing to heal, and sedges and willows are firmly established. Though juniper tends to resist decay and has a more desirable structure for stream revetment purposes, Jeffrey Pine held up remarkably well after ten years of exposure and is therefore recommended in the absence of juniper.
• Willow matting installed at the south bank of Pond #1 did not halt erosion. The mats were probably washed out by high flows before they had a chance to take root and develop sufficiently to provide erosion protection. The matting was prepared before willow stakes were dormant, possibly causing stress. In addition, return flow from the overtopping of Check Dam #4 reentered the channel on top of the matting (between Check Dams #1 and #2), undermining the matting (see Figure 3-3). Variations of this technique have proved successful at other projects in the watershed. Such structures must be designed and sited with care to maximize success.

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