When is stream invertebrate drift catastrophic? The role of hydraulics and sediment transport in initiating drift during flood events

Abstract

1. The term 'catastrophic drift' is used to describe the large-scale displacement of invertebrates that occurs during periods of increased river discharge. However, the physical processes that lead to animals entering the water column at such times remain poorly understood. Specifically, the hypothesis that the movement of bed sediments during floods triggers a large increase in drift has lacked a rigorous field test. 2. Using a portable flume, the hydraulic conditions and rates of bedload transport associated with small, frequent floods were created in situ within a reach of a gravel bed river. Experiments focussed on the patches of fine sediment which are the dominant source of bed material transported during small floods. The flume produced near bed velocities of up to 2 m s )1 over the patches, increasing shear stress, initiating sediment transport and causing invertebrates to enter the drift. 3. The total number of individuals lost from the bed, as well as the taxonomic composition of the drift, were influenced strongly by shear stress and bedload. The rate of loss from the bed was low at shear stresses <9 dynes cm )2 (0-4 individuals min )1 from the 0.5 m 2 flume bed area). Once shear stress exceeded 9 dynes cm )2 , the threshold that resulted in consistent bedload transport from the patches, the rate of loss of animals increased to a maximum of 56 individuals min )1 . When bedload transport rates were at their highest, the taxonomic composition of the drift was more similar to the benthos than it was to the drift observed when bed material was stable. 4. Absolute rates of bedload transport created by the manipulations were extremely low (<7 g m )1 s )1 ) and typical of those measured during small, frequent floods. Events of this magnitude do not break up the armour layer across the reach as a whole and so exposed patches of fine sediment are the principal source of bedload material. Consequently, discharge events not considered as disturbances in geomorphic terms may initiate frequent episodes of so-called 'catastrophic drift' from patches of stream bed.

Causes and consequences of invertebrate drift in running waters: from individuals to populations and trophic fluxes

“…First, the critical level of shear stress where organic matter becomes entrained increases the passive drift of animals using detritus or algal mats as substrate (Vinson 2001). A second threshold is reached as saltation of sand and fine organic matter scour exposed benthic invertebrates (Gibbins et al 2007a), followed by a third threshold at bed-mobilizing flows as surface and nearsurface invertebrates are entrained during mobilization of all particles on the stream bed (Anderson and Lehmkuhl 1968). However, substrate mobilization may not be a crucial condition for mass drift if the critical shear stress for substrate entrainment exceeds that for invertebrates.…”

“…However, substrate mobilization may not be a crucial condition for mass drift if the critical shear stress for substrate entrainment exceeds that for invertebrates. In this case, catastrophic drift may be frequently initiated by high discharge events not considered disturbances in geomorphic terms (Statzner et al 1984;Gibbins et al 2007a).…”

“…Catastrophic drift is the most obvious case where hydraulic forces cause involuntary entrainment and transport of both invertebrates and bed particles. For example, (Gibbins et al 2007a) found a clear threshold of shear stress at which catastrophic drift entry was initiated, where catastrophic drift was defined as rapid increases in suspended invertebrate abundance and a compositional shift in drifting taxa to resemble that of the benthos. Identifying a causal mechanism for drift at flows below a catastrophic threshold is more difficult and requires careful experimentation.…”

“…"Catastrophic drift" or "mass drift", defined as a rapid increase in passive drift (Anderson and Lehmkuhl 1968;Gibbins et al 2007a), may result from several flow-related thresholds. First, the critical level of shear stress where organic matter becomes entrained increases the passive drift of animals using detritus or algal mats as substrate (Vinson 2001).…”

Animal perception in gravel-bed rivers: scales of sensing and environmental controls on sensory information

“…Benthic animals can be dislodged or damaged by moving bed material. When sediment transport occurs, even at low to moderate levels, the number of invertebrates found drifting in the water column increases greatly (Gibbins et al 2007). However, river beds remain stocked with invertebrates post-flood, implying that many organisms can survive such events by actively or passively finding refuge (Lancaster and Hildrew 1993;Gjerløv et al 2003;Death 2006).…”