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

1. The hyporheic zone has long been regarded as a potential refugium for lo tic invertebrates during disturbance. However, there have been few attempts to quantify the stability of this habitat during high flow events. In a Ne w Zealand stream with an unstable bed, the present authors monitored spatia l patterns of scour and fill in a riffle in a wide flood plain and at two s ites in a constrained reach: a pool-riffle with bedrock outcrops and a plan e-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 o f chain exposed before and after a high flow event, whereas filling depth w as equivalent to the thickness of the sediment deposited on top of the chai ns during the event. For each chain, the present authors noted dominant par ticle size and degree of packing of the surrounding bed, water depth and pr esence or absence of large stones upstream. Chains were re-located after fo ur smaller spates, one intermediate event and one large flood. 3. Most events caused a complex mosaic of bed patches which experienced sco ur, fill or remained undisturbed. These patterns, which were mostly site- a nd 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 differenc es 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 re duced near-bottom shear stress during larger events and led to sediment dep osition. 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 hyporhei c zone below this depth would have been available as refugium for invertebr ates, in addition to the often considerable number of bed patches which rem ained 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.