FINE SEDIMENT INFILTRATION INTO GRAVEL SPAWNING BEDS WITHIN A REGULATED RIVER EXPERIENCING FLOODS - ECOLOGICAL IMPLICATIONS FOR SALMONIDS

Infiltration rates into eight gravel spawning beds were monitored over a range of discharges including compensation flows, peak hydropower generation and floods of up to bankfull capacity. The results indicate that the interrelationship of sediment supply and discharge influence both the rate and grain size composition of infiltrated sediments, with maximum rates experienced during bankfull floods when sediments are scoured from upstream pools. The effects of river regulation for hydropower are shown to produce a finer matrix infill in the absence of unregulated tributary sources, although the rates of infiltration are much lower than for sites downstream of unregulated tributaries. Values for infiltration rates are shown to be detrimental for small:salmonid spawning redds during post-flood conditions when sediment transport is not restrained by supply.