Range of variability of channel complexity in urban, restored and forested reference streams

Abstract

Summary 1. Channel complexity is an important ecological property of stream systems and is often targeted for restoration in channelised urban streams. However, channel complexity is rarely defined explicitly, and little research on channel complexity has been conducted in streams in urban catchments that have not been directly channelised by human activities. Therefore, it remains unclear whether restoration of non‐channelised urban streams has improved complexity. 2. We explicitly define channel complexity and use a multimetric approach to provide a comprehensive assessment of complexity in multiple restored, urban and forested streams on the Maryland Coastal Plain and two streams of differing land use in Colorado. We also expand on the Maryland and Colorado results with a literature survey of channel complexity from diverse geographical regions. 3. Many streams draining urban catchments in Maryland had relatively high values of some complexity metrics compared to forested reference streams in Maryland and compared to the values for pristine streams calculated from the literature. This suggests that streams in urban catchments that are not directly manipulated by human activities (e.g. channelisation or piping) may be able to maintain channel structures beneficial for aquatic organisms even when impervious surfaces are the dominant form of land use in the catchment. 4. Restored streams in Maryland had equal or lower values of many complexity metrics compared to streams draining urban catchments in Maryland. This suggests that restoration of streams draining urban catchments did not improve the overall channel complexity. 5. Our results highlight the need to explicitly define and measure the attributes of channel complexity that are targeted during restoration, to determine whether the streams in urban catchments are truly degraded with respect to channel complexity. 6. Combined with recent synthesis work suggesting that biodiversity may not be improved by increasing the channel complexity, these results indicate that targeting catchment processes may prove a more useful approach to restoration than attempting to move channel complexity in streams draining urban catchments towards conditions in forested reference streams.

Evaluating Stream Restoration Projects: What Do We Learn from Monitoring?

“…This issue is further complicated by the absence of clear, agreed-upon, definitions of how to define and measure habitat heterogeneiity. The importance of quantitatively and precisely assessing habitat heterogeneity was demonstrated by Laub et al [49] who found that many unrestored (but non-channelized) urban streams had relatively high heterogeneity (measured through several specific metrics including variability in width, depth, velocity, thalweg profile, and bed sediment sorting) when compared to reference, forested sites, and that "restored" sites were often not more complex than unrestored sites. Thus, there is no a priori reason to assume that "heterogeneity enhanced" sites have more heterogeneity than unrestored urban stream sites.…”

Variation of Hyporheic Potential among Urban Region Streams: Implications for Stream Restoration

“…We therefore picked sinuosity (S) as our metric of planform complexity. By contrast, many metrics that capture some aspect of vertical complexity of the thalweg have been proposed (Laub et al, 2012). The most common metric is the density/spacing of instream geomorphic features, yet this metric does not incorporate information on the vertical height of bed deviations.…”

“…For example, the spatial variability of shallow sediment hydraulic conductivity (K) has been measured within one or two streams (e.g., Ryan and Packman, 2006;Ryan and Boufadel, 2007;and Kennedy et al, 2009a,b). Similarly, Laub et al (2012) examined how restoration in urban areas affects channel ''complexity,'' which is defined in many ways but includes variations in streambed topography that drive hydraulic head gradients that drive bidirectional exchange. The relationship between hyporheic potential and hyporheic exchange is discussed further in ''Hyporheic Potential Versus Hyporheic Exchange,'' below.…”

Urban stream renovation: incorporating societal objectives to achieve ecological improvements

“…1; Choi 2007, Violin et al 2011, Laub et al 2012, Hawley et al 2013. Streams in urban landscapes often have irreversibly altered physical characteristics at small-to-broad spatial scales that prevent a return to natural hydrogeomorphic characteristics of reference conditions (e.g., unattainable channel slopes because of permanently altered local topographies; Wohl et al 2005).…”

“…From an ecological perspective, consistently effective strategies for restoring streams in urban catchments are elusive , Violin et al 2011, Laub et al 2012). The primary ecological barriers for restoring streams in modified landscapes is the extent to which drivers of stream condition (e.g., flow, sediment supply, water quality, habitat availability, etc.)…”