The influence of riparian vegetation on stream width, eastern Pennsylvania, USA

Abstract

We surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks. Cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches, while deposition is localized on active fl oodplains on the insides of bends.Our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches. Two dimensionless parameters, α and E, explain our observations. α represents the infl uence of grassy vegetation on rates of active fl oodplain deposition; it is 5 times higher in nonforested reaches than in forested reaches. E is proportional to rates of cutbank migration; it is 3 times higher in nonforested reaches than in forested reaches. Differences in width between forested and nonforested reaches are proportional to E/α. In forested reaches, channels are wide with banks that are diffi cult to erode. Dense tree roots create a low value of E, and the channel migrates slowly. E/α is high, however, because α is very low: shade from trees inhibits the growth of grass on active fl oodplains. In nonforested reaches, channels are narrow with banks that are easy to erode. E is high, and the channel migrates rapidly. E/α is low, however, due to a very large value of α: grass grows readily on nonforested convex bank fl oodplains. Thus, differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active fl oodplains, implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetationmediated rates of deposition on active fl oodplains. These results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and fl oodplain depositional processes, rather than focusing on these processes in isolation from one another.

Case Study: Kerber Creek Restoration Project: Employing Statistical Techniques to Analyze Effects of Restoration Activities, Saguache, Co

“…While there are some studies that report the effects of riparian vegetation on channel morphology (e.g., Allmendinger et al, 2004), the failure to reject the null hypothesis for the phytostabilization index is generally consistent with similar studies in the literature. For example, Hession et al (2003) report no significant difference in sinuosity between forested and nonforested stream reaches, indicating that changes in the density of riparian vegetation do not significantly affect sinuosity.…”

The influence of a vegetated bar on channel-bend flow dynamics

“…Initial riparian forest development may result in a decrease in width-to-depth ratio as formerly bare banks are vegetated and increase bank cohesion, preventing bank erosion from widening channels (Métivier and Barrier, 2012) such that meanders (Eaton and Giles, 2009) and alternate bars emerge (Kleinhans, 2010). For 15 channels characterized by vegetated banks and meandering planforms, differences in width have been observed based on floodplain and bank vegetation type, with floodplains composed of herbaceous vegetation associated with narrower channels compared to those composed of woody vegetation (Allmendinger et al, 2005;Hession et al, 2003;Jackson et al, 2015). It is unclear what would cause this relationship, since bank strength increases with rooting depth (Eaton and Giles, 2009), which is greater for woody vegetation compared to herbaceous vegetation (Canadell et al, 1996) …”

“…A larger velocity gradient within the thalweg compared to over the bar would be expected to alter the dynamics of bank erosion. As a simple rule, bank erosion rate, ṅ, according to Parker et al's (2011) HIPS model is proportional to an erosion coefficient, k, and half the streamwise velocity difference between the two banks, Δu: Vegetation "pushing" flow toward the outer bank is analogous to "bar push" (Allmendinger et al, 2005;Parker et al, 2011), whereby a rapidly accreting point bar may cause erosion at the outer bank (Eke et al, 2014;van de Lageweg et al, 5 2014). This increase in bank erosion would be countered by deposition of fine sediment on the bar resulting from the vegetation-induced reduction in velocity in this region, that may in turn induce addition "push" through bar building (e.g., Eke et al, 2014).…”

Vegetative impacts upon bedload transport capacity and channel stability for differing alluvial planforms in the Yellow River source zone

“…Bar development and stability reflect the ability of vegetation to trap sediments and stabilize banks, which in turn is directly influenced by flow energy relationships (i.e., these are mutual adjustments; Corenblit et al, 2007;Gurnell et al, 2012;Gurnell, 2014;Osterkamp and Hupp, 2010;Pietsch and Nanson, 2011). In this study, riparian vegetation and its root network are considered to restrict channel width and increase hydraulic efficiency, inducing greater bedload transport capacity in multi-thread channels (Allmendinger et al, 2005;Huang and Nanson, 2007). Islands and floodplains are able to trap more fine-grained sediment in the flood season, enhancing the longer-term (decadal) stability of anabranching channels, as shown by the stable islands of the Maqu reach.…”