Watersheds and Vegetation

Adapted from: Mary Nichols. 2007. Chapter 4: Stream processes in riparian areas. In: G. Zaimes (ed). Understanding Arizona’s Riparian Areas. University of Arizona Cooperative Extension. AZ 1432. Available at http://cals.arizona.edu/pubs/natresources/az1432.pdf

Figure 1. Vegetation found in watersheds can influence channel formation. Trees and fine-stemmed vegetation, such as grasses, influence how water flows, sediment deposition, and channel roughness. Photo by Chris Evans, River to River CWMA. Image courtesy of forestryimages.org.

The relatively dense stands of vegetation found along channels form in response to available moisture in watersheds. Vegetation typically colonizes channel floodplains and banks, and in the absence of scouring flood flows, can become established on the channel bed. Vegetation, both on channel banks and within channels, can play an important role in controlling morphologic adjustments of channels by altering resistance to erosion and affecting flow hydraulics. In extreme cases, riparian vegetation can act as a primary control on channel shape (Tal et al. 2003). Because of its importance in affecting channel morphology, vegetation can be used as a beneficial tool for managing riparian areas.

Vegetation can act as a stabilizing force. On the floodplain and along channel banks, roots provide a network of reinforcement to bind the soil matrix and increase soil strength (Simmon and Collison 2002). There is a wide range of rooting depth among riparian species. The roots of woody vegetation may extend several feet while the rooting depth of some grasses may not exceed several inches. The range in rooting characteristics leads to a range in the stabilizing forces of riparian plants.

Although intense flood flows can scour, uproot, and remove young and newly established vegetation, established vegetation can act to stabilize channel bed sediment that would otherwise be readily mobilized. As the vegetation matures, it becomes increasingly resistant to removal during flood flows.

Additionally, vegetation interacts directly with flowing water. Because vegetation imparts a resistance as water flows past stems and through leaves and branches, it slows the flow and affects the pattern of erosion and deposition along the channel. The relatively stiff stems of woody vegetation may create high turbulence as flow travels around stems and produces local pockets of erosion. Grasses and finer-stemmed vegetation may simply bend as flow passes over them, thus contributing to channel roughness. Vegetation can also act as a filter, promoting deposition as sediment-laden water passes.

Adapted for eXtension.org by Sabrina Kleinman, University of Arizona

References Cited
Simon, A. and A.J.C. Collison. 2002. Quantifying the mechanical and hydrologic effects of riparian vegetation on streambank stability. Earth Surface Processes and Landforms. 27: 527-546.

Tal, M.K., K. Gran, A.B. Murray, C. Paola, and D.M. Hicks. 2003. Riparian vegetation as a primary control on channel characteristics in multi-thread rivers. Riparian Vegetation and Fluvial Geomorphology: Hydraulic, Hydrologic, and Geotechnical Interaction. Water Science Application. Vol. 8. American Geophysical Union.

For more on Watersheds and Vegetation:

• Watersheds and Forests
• Watersheds and Channel Morphology
• Watersheds and Floodplains
• Processes that Shape Watersheds
• How Watersheds Change Over Time

Additional Resources:

• Riparian Forest Buffers – University of Nebraska–Lincoln
• Riparian Buffers – North Carolina State University
• Riparian Forest Buffers – USFS Northern Area
• [PPT] Riparian Forest Buffers – Purdue Agriculture