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Channel Processes: Streambank Erosion

Streambank erosion is a natural process, but acceleration of this natural process leads to a disproportionate sediment supply, stream channel instability, land loss, habitat loss and other adverse effects. Streambank erosion processes, although complex, are driven by two major components: stream bank characteristics (erodibility) and hydraulic/gravitational forces. Many land use activities can affect both of these components and lead to accelerated bank erosion. The vegetation rooting characteristics can protect banks from fluvial entrainment and collapse, and also provide internal bank strength. When riparian vegetation is changed from woody species to annual grasses and/or forbs, the internal strength is weakened, causing acceleration of mass wasting processes. Streambank aggradation or degradation is often a response to stream channel instability. Since bank erosion is often a symptom of a larger, more complex problem, the long-term solutions often involve much more than just bank stabilization.

Erosion rates can sometimes be estimated from sequential, time-trend aerial photos on larger rivers. Lateral erosion rates from aerial photos and field measured bank heights have been used to estimate sediment production rates (Lehre et al. 1983). Aerial photographs were also used to compare various riparian vegetative types to bank retreat rates (Pizzuto and Mecklenburg 1989). Annual bank erosion rates were increased by three orders of magnitude due to willow eradication and conversion from woody species to a grass/forb riparian community on Wolf Creek, Colorado (Rosgen 2001a).

Numerous studies have demonstrated that streambank erosion contributes a large portion of the annual sediment yield. Over 90 percent of the total suspended sediment yield during bankfull discharge of a snowmelt runoff event on the West Fork of the Madison River, Montana was associated with unstable streambanks and channel instability (Rosgen 1973, 1976). Simon (1989) reported lateral bank erosion rates from the Forked Deer River system in West Tennessee at 1.5 m/yr representing a bank erosion contribution of 82 percent of the 10 million tons/year, with 18 percent contributed by bed degradation. Streambank erosion rates of 14 m/yr were measured in the Cimmaron River in Kansas (Schumm and Lichty, 1963), 50 m/yr., in the Gila River, Arizona and 100 m/yr on some reaches of the Toutle River, Washington (Simon 1992).

Streambank erosion contributed 49 percent of the total annual sediment yield from three miles of unstable channel on the East Fork San Juan River, Colorado (Rosgen, 2001a). The accelerated sediment supply contributing to the annual sediment budget was caused by a conversion from a stable C4 stream type (meandering, width/depth ratio of 25, gravel bed river with a well developed floodplain) to an unstable D4 stream type (gravel bed braided channel with a width/depth ratio of 250). The stream type conversion and associated instability was caused by willow eradication in the 1930s (Rosgen, 2001a). The "river pedestals" of the East Fork of the San Juan River shown in Figure 28 and Figure 29 indicate the high rate of lateral erosion. The pedestals are remnants of the streambank of the river terrace left standing approximately 4 feet above the gravel bar.

Figure 28

Figure 28. "River pedestals" of the East Fork San Juan River, remnant of the previous river terrace bank, indicating high rate of lateral erosion.

Figure 29

Figure 29. "River pedestals" of the East Fork San Juan River, remnant of the previous river terrace bank, indicating high rate of lateral erosion.

Due to willow removal from herbicide spraying conducted 18 years earlier, approximately 60 feet of streambank was eroded between 1995-1998 on Wolf Creek, Colorado, during two major floods. Concurrent measurements of an upstream, untreated C4 stream reach resulted in negligible rates of erosion during the same period. As a result of the willow removal, the width of the downstream reach was increased from 30 to 198 feet and the channel was enlarged with the corresponding cross-sectional area of the bankfull discharge from 55 ft.2 to 158 ft.2. The adjustments in the stream channel as a result of the streambank destabilization by riparian vegetation change and subsequent accelerated erosion were such to create a D4 (braided) channel. The consequence of the stream type change was to:

  • Increase sediment supply from streambank erosion
  • Decrease shear stress
  • Decrease unit stream power
  • Reduce sediment transport capacity
  • Decrease competence (ability to move largest sized sediment made available)
  • Fill in pools, and created bi-modal bed material size
  • Decrease fish habitat
  • Create major land loss
  • Result in loss of visual value

Examples of the C4 stream type that existed prior to the willow spraying are shown in the aerial photo taken in 1976, (Figure 30). The contrast replicate aerial photo taken in 1991 is shown in Figure 31.

Figure 30

Figure 30. Aerial photograph (1976) of Wolf Creek, Colo. showing C4 stream type prior to spraying.

Figure 31

Figure 31. Aerial photograph (1991) showing change in Wolf Creek to a D4 stream type, but little change in the upstream, untreated, (above fence line) C4 stream type. Flow is left to right.

An example of the excessive streambank erosion caused by a conversion of willows to a grass/forb riparian community is shown in Figure 32. Change in the rooting depth and density made the banks more susceptible to fluvial entrainment, triggering upper bank collapse. The lack of sediment transport capacity resulted in excess deposition, re-directing high shear stress to the near-bank region and causing accelerated stream bank erosion. The deposition of sediment from upstream sources, as well as increased supply from streambank erosion, both add to the long-term instability of Wolf Creek.

Figure 32

Figure 32. Typical eroding bank on the D4 (braided) reach of Wolf Creek following willow removal from herbicide spraying.

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