Reach‐Scale Geomorphic and Biological Effects of Localized Streambank Armoring

Abstract

Armoring of streambanks is a common management response to perceived threats to adjacent infrastructure from flooding or erosion. Despite their pervasiveness, effects of reach-scale bank armoring have received less attention than those of channelization or watershed-scale hydromodification. In this study, we explored mechanistic ecosystem responses to armoring by comparing conditions upstream, within, and downstream of six stream reaches with bank armoring in Southern California. Assessments were based on four common stream-channel assessment methods: (1) traditional geomorphic measures, (2) the California Rapid Assessment Method for wetlands, (3) bioassessment with benthic macroinvertebrates, and (4) bioassessment with stream algae. Although physical responses varied among stream types (mountain, transitional, and lowland), armored segments generally had lower slopes, more and deeper pools and fewer riffles, and increased sediment deposition. Several armored segments exhibited channel incision and bank toe failure. All classes of biological indicators showed subtle, mechanistic responses to physical changes. However, extreme heterogeneity among sites, the presence of catchment-scale disturbances, and low sample size made it difficult to ascribe observed patterns solely to channel armoring. The data suggest that species-level or functional group-level metrics may be more sensitive tools than integrative indices of biotic integrity to localscale effects.

Does DNA barcoding improve performance of traditional stream bioassessment metrics?

“…Our study streams were subjected to mild forms of degradation that resulted in subtle changes in the benthic community (Stein et al 2013). Bank armoring affects channel substrate indirectly and leads to a higher percentage of fine sediments and higher pool density, but the substrate remains largely natural.…”

“…Bank armoring affects channel substrate indirectly and leads to a higher percentage of fine sediments and higher pool density, but the substrate remains largely natural. The effect of these changes on the biota is generally too subtle to be detected with richnessbased metrics and can be discerned only when investigating functional traits of BMIs in affected reaches (Stein et al 2013). However, with a few exceptions, we currently lack information on functional traits, pollution tolerances, or niche preferences of many of the "new" species identified through barcoding.…”

“…Armored segments (B) were most often along the outside of meander bends. In all cases, bank armoring structures (which severely constrained the lateral movement of the study streams) enhanced the forcing of pool scour and meander development by constraining channel migration (Stein et al 2013). This assertion is supported by observations of historical planform photographs of the sites, which show that all of the channels had been quite dynamic, both upstream of and within the bank segments that were eventually armored.…”

“…We took advantage of a companion study in which the effects of bank armoring were evaluated in 5 paired stream reaches, 1 armored and 1 unarmored reach in each location (Stein et al 2013). Eroding stream banks are often reinforced (armored) with hard structures, such as concrete walls, boulders, or gabions.…”

“…Eroding stream banks are often reinforced (armored) with hard structures, such as concrete walls, boulders, or gabions. These bank-armoring structures are intended to stabilize channels and protect infrastructure, but they often result in increased stream power and decreased channel roughness, which exacerbate stream-channel responses to urbanization (Riley 1998, Jacobson et al 2001 (Stein et al 2013).…”