Historic influences on modern bank erosion: A case study in Crabbs Branch, Maryland

Streams are complex systems that continually evolve to accommodate changes in the amounts of water and sediment supplied from the watershed. Land use changes in a watershed can have large impacts on watershed hydrology and channel morphology in stream networks, often taking decades to resolve. As a result, it is useful and important to consider the watershed history when attempting to understand why a channel is degraded. A case study of an impaired study reach of Crabbs Branch is an example of how geomorphic processes observed today can be indicative of an ongoing process of adjustment and readjustment related to land use changes in the watershed.

Crabbs Branch is a first order stream located within the Upper Rock Creek watershed of central Montgomery County, Maryland (Figure 1). Crabbs Branch is the first tributary to Rock Creek downstream of Lake Needwood, and accordingly is the head of the free-flowing portion of the Rock Creek watershed. The study reach is located within the Derwood neighborhood near the Shady Grove Metro station (Figure 2) and includes 5,365 linear feet of Crabbs Branch. Land use surrounding the study reach is primarily medium-density residential, institutional, and industrial land. The drainage area at the downstream extent of the study reach is approximately 1.9 square miles and consists of approximately 40 percent impervious area (GISHydro2000, 2006). The Crabbs Branch study area has been classified as a gravel-bed, pool-riffle stream with equiwidth meanders (Montgomery and Buffington, 1997). Small drainage networks flow across the floodplain, including a side channel that siphons flow from the main channel during high water stages and rejoins the channel further downstream.

Existing conditions at the Crabbs Branch study reach are the result of an uncertain and complicated history of channel impacts including channel straightening, construction of infrastructure, changes in watershed hydrology and sediment load from agriculture and urbanization, and beaver impacts to channel morphology and riparian vegetation assemblages. Bank slumping is prevalent along the channel in most of the study reach. This is a problem because fine sediment stored in the floodplain enters the stream channel as the banks erode, contributing to high embeddedness (the degree to which gravels are covered in silt) and reduced benthic habitat. The invertebrate groups that drive good biotic index scores (mayflies, stoneflies, and caddisflies) are essentially absent from Crabbs Branch. These species depend on good water quality, limited suspended sediment, and low embeddedness.

The cut banks show a buried, organic and clay rich soil close to the base water stage of Crabbs Branch that may be indicative of a pre-colonial surface (Figure 3). A tractor tire and three logs are buried in the banks along the study reach at depths of 2.3, 2.8 and 3.0 feet. These buried objects indicate that the floodplain has aggraded through time and that the floodplain elevation is higher than it was in the past. Jacobson and Coleman (1986) have documented that sediment loss and runoff from post-colonial agricultural fields increased sediment supply to stream networks, which has been partially stored in thick floodplain sediments deposited during out-of-bank stream discharges. As early as 1840, the agricultural fields of central Maryland were severely degraded and depleted due to intensive corn and tobacco farming, and many people were leaving the area for better farmlands to the west (Montgomery County, 1999). Historic information indicates that from the mid-19th Century until the late 1970's the primary land use within the Crabbs Branch watershed was agriculture with isolated patches of forest located between fields (Martenet, 1865; Montgomery County Aerial Photographs, 1951, 1970, 1979).

Field observations indicate that mass wasting (slumping of undercut banks) and subaerial processes (freeze-thaw cycles) have a large influence on bank erosion rates at Crabbs Branch, possibly more so than typical fluvial processes such as scour. Bank undercuts seem to largely develop during the winter months, and thawing banks on a warming winter day were observed dropping small pieces of bank onto the surface of the frozen stream (Figure 4). Wynn et al. (2008) determined that soil erodability in winter is more than twice that in the summer due to the effect of freeze-thaw cycles. This effect is more pronounced in agricultural legacy sediments which are fine-grained, and banks with a silt and clay content greater than 20% are considered to be "frost-susceptible" (Wynn et al., 2008).

The hydrology and morphology of Crabbs Branch in the study reach have been heavily influenced by anthropogenic structures. The Crabbs Branch Regional Stormwater Pond is an approximately 21 acre stormwater facility located in the headwaters of Crabbs Branch that captures runoff from approximately 48 percent of the study reach drainage area. As the Crabbs Branch Regional Stormwater Pond stores and delays runoff from those areas upstream of it, the highly urban area between Redland Road and the project site dominates the hydrology of Crabbs Branch throughout the RC-74 study area. The high percentage of impervious area in this sub-watershed has led to flashy runoff typical of urban drainage areas, which is most evident on the rapid rising limb of the measured stage hydrograph (Figure 5). Quantity control provided by the stormwater pond prolongs the time of high stage in Crabbs Branch, lengthening the falling limb of the hydrograph to up to 3 days in some events. The Crabbs Branch Regional Stormwater Pond is providing attenuation of flow during large storm events, but still discharging at relatively high flows for long periods of time, which has been shown to cause excessive shear stress (a measure of the stream's ability to mobilize sediment) for longer durations and increased stream channel erosion (McCuen et al., 1987).

Meander lengths, belt widths and radius of curvatures in the Crabbs Branch study reach are small compared to the values expected for stable stream channels (Williams, 1986), which also causes large amounts of bank erosion as the stream adjusts laterally to a more stable form. It has been measured that sinuosity increased from historic levels after installation of the Regional Stormwater Pond as Crabbs Branch adjusted to the new hydrologic regime (Figure 6). Additionally, buried infrastructure crosses the stream channel in the study area, including six water line crossings, one sewer line crossing, and one gas line crossing. The stream channel alignment has been altered and the stream bed hardened at most utility crossings. The lateral instability may also be a result of ongoing adjustment to these areas of channelization.

Natural disturbances in a watershed can also lead to channel instability and bank erosion. The 1993 aerial photograph shows a number of beaver dams in Crabbs Branch, including two that occupy most of the valley width (Figure 7). The establishment of the side channels appears to have resulted predominantly from the beaver activity, either from flooding of a previously existing small tributary, scour from flow out of the larger beaver pond upstream, or excavation by the beavers as a 'beaver canal.' The 1993 aerial photograph is also the first to show the presence of wetlands and wet areas on the floodplain. The Crabbs Branch channel was widest in 1993 due to backwater effects from the beaver dams. The large difference in floodplain features between 1979 and 1993 suggest that the beaver activity had a significant influence on geomorphic processes in Crabbs Branch. It is possible that lateral instability in the study reach is also a legacy of the impacts of the beaver dams, over 14 years later.

In conclusion, modern problems with excessive bank erosion in Crabbs Branch can be attributed to these combined influences:
- Historic aggradation of the floodplain resulting from agricultural practices in the watershed
- Excessive erosion due to freeze-thaw cycles prevalent in fine grained legacy sediments
- Excessive shear stress resulting from the urbanized watershed hydrology
- Lateral planform adjustment in response to channelization, hydrology changes, and beaver impacts

Funding for this study was provided by the Maryland State Highway Administration.

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