Scour and fill patterns in a New Zealand stream and potential implications for invertebrate refugia

Abstract

1. The hyporheic zone has long been regarded as a potential refugium for lotic invertebrates during disturbance. However, there have been few attempts to quantify the stability of this habitat during high flow events. In a New Zealand stream with an unstable bed, the present authors monitored spatial patterns of scour and fill in a riffle in a wide flood plain and at two sites in a constrained reach: a pool‐riffle with bedrock outcrops and a plane‐bed (a bedform characterized by long stretches of planar stream bed).2. At each 20‐m site, 100 scour chains were installed in a systematic grid with about 1 m between chains. Scour was measured by comparing the length of chain exposed before and after a high flow event, whereas filling depth was equivalent to the thickness of the sediment deposited on top of the chains during the event. For each chain, the present authors noted dominant particle size and degree of packing of the surrounding bed, water depth and presence or absence of large stones upstream. Chains were re‐located after four smaller spates, one intermediate event and one large flood.3. Most events caused a complex mosaic of bed patches which experienced scour, fill or remained undisturbed. These patterns, which were mostly site‐ and event‐specific, were often significantly influenced by the longitudinal or lateral position of the chains in the spatial grids.4. The cumulative effect of the six high flow events differed substantially between sites. The first site experienced predominantly scour, the second both scour and fill, and the third almost exclusively fill. These differences were partly explained by channel geomorphology. The bedrock outcrops at the constrained pool‐riffle site forced the flow at high discharge, causing deep scour in these areas, whereas a backwater effect at the third site reduced near‐bottom shear stress during larger events and led to sediment deposition.5. Except for a single event at the second site, scour affected mainly the uppermost 10–15 cm of the stream bed. Therefore, almost the entire hyporheic zone below this depth would have been available as refugium for invertebrates, in addition to the often consider‐able number of bed patches which remained undisturbed during the six high flow events.6. Fill without earlier scour during the same high flow event was common at all sites. Most previous studies have assumed that lotic invertebrates are mainly affected by scour during high flow events, but the consequences of sediment deposition may be just as far reaching.