Modeling the effects of pulsed versus chronic sand inputs on salmonid spawning habitat in a low‐gradient gravel‐bed river

Abstract

It is widely recognized that high supplies of fine sediment, largely sand, can negatively impact the aquatic habitat quality of gravel-bed rivers, but effects of the style of input (chronic vs. pulsed) have not been examined quantitatively. We hypothesize that a continuous (i.e. chronic) supply of sand will be more detrimental to the quality of aquatic habitat than an instantaneous sand pulse equal to the integrated volume of the chronic supply. We investigate this issue by applying a two-dimensional numerical model to a 1 km long reach of prime salmonid spawning habitat in central Idaho. Results show that in both supply scenarios, sand moves through the study reach as bed load, and that both the movement and depth of sand on the streambed mirrors the hydrograph of this snowmelt-dominated river. Predictions indicate greater and more persistent mortality of salmonid embryos under chronic supplies than pulse inputs, supporting our hypothesis. However, predicted mortality varies both with salmonid species and location of spawning. We found that the greatest impacts occur closer to the location of the sand input under both supply scenarios. Results also suggest that reach-scale morphology may modulate the impact of sand loads, and that under conditions of high sand loading climaterelated increases in flow magnitude could increase embryo mortality through sand deposition, rather than streambed scour.

Controls on spatial and temporal variations in sand delivery to salmonid spawning riffles

“…This association is shown more clearly in Figure , where infiltration rate is plotted against discharge. When the mean data is considered, a strong dependence between total deposition and discharge was shown (r 2 = 0.7, p < .01), a finding also noted by Maturana et al (). Extrapolation of the regression equation suggested that fine bedload movement was initiated at discharges in excess of 0.7 m 3 s −1 .…”

“…The clear association between infiltration rates and discharge, with greatest rates of infiltration being associated with high winter flows is a feature supported by other investigations (Maturana et al, ). Higher winter discharges have the competence to transport greater quantities of bedload; however, the supply of sand is also a key factor.…”

“…Furthermore, the 2–4 mm fraction was found to show enhanced infiltration rates in high velocity zones in the pool and a number of other high velocity threads across shallower areas. Maturana et al's () modelling study also suggests that fine sediment can be steered away from pools, tending to infiltrate point bars and shielding downstream riffle spawning habitat from sedimentation. Welton () has also shown spatial variation for Tadnoll Brook, United Kingdom, a lowland chalk stream, with 5kgm −2 day −1 deposited along the thalweg and up to 70kgm −2 day −1 deposited along the channel margins.…”

Using Natural Disturbance and Portfolio Concepts to Guide Aquatic–Riparian Ecosystem Management

“…This may involve mimicking the underlying process through novel disturbances (e.g., cutting trees to simulate wildfire loss that initiates stand dynamics but also leaving the forest to recover) or restoring natural disturbance regimes. Human actions interrupt migratory life histories of aquatic organisms through road crossings and other barriers (Dunham et al 2003;Rieman et al 2003); alter sediment production and transport regimes (Istanbulluoglu et al 2004;Goode et al 2012;Maturana et al 2014); change flow, bed scour, and temperature regimes (Marks et al 1998;Lessard and Hayes 2003;Tonina et al 2008); and alter wood loading and channel morphology (Montgomery et al 1995;Wood-Smith and Buffington 1996;Benda et al 2003;May and Gresswell 2003). For example, fire-related management activities may disrupt watershed processes and degrade habitat for sensitive fishes (Rieman et al 2010).…”

“…Allowing natural disturbance processes to play out, however, may sometimes be unfeasible because of other societal demands on forests and water. For example, chronic supplies of fine sediment from forest roads can have a greater and longer-lasting impact on salmonid spawning habitats than an equal volume of material introduced as a sediment pulse by a naturally occurring debris flow or landslide (Maturana et al 2014). Human influences offer a means of promoting disturbance and biological response but frequently differ from natural disturbances in the timing, magnitude, spatial extent, and nature of the disturbance (Lindenmayer and Franklin 2002;Hessburg et al 2005), and may result in ecosystem structure and connectivity that differ from natural conditions (Stanford and Ward 1992).…”