Assessment of streambed stability in steep, bouldery streams: development of a new analytical technique

Substrate stability in streams is fundamental to benthic communities, many of which decline in biomass and taxon richness with decreased stability. Al though of obvious importance in describing a stream's disturbance regime, s ubstrate movement is often difficult to quantify accurately. Traditional te chniques rely either on subjective scoring systems (e.g, Pfankuch stability score), or require repeated site visits to follow downstream displacement of painted stones. Newbury's Instability Index measures substrate movement based on calculations of a stream's tractive force at bankfull discharge, b ut the assumptions on which this method is based limit its use to lowland s treams. We have developed a new method to assess substrate movement suitabl e for shallow, steep mountain streams with a high bed roughness, based on a n extensive review of existing hydraulic literature We calculated the % of the streambed in motion at bankfull discharge (P-BF) in 48 New Zealand head water streams using our method, the Instability Index, and Pfankuch score. Concordance analysis showed that the 3 methods of assessing substrate stabi lity ranked the 48 streams differently. We next related the 3 stability mea sures to aquatic bryophyte cover because these plants are good biological i ndicators of substrate stability. We observed a highly significant correlat ion (p < 0.001) between P-BF and bryophyte cover, a weaker correlation betw een the Pfankuch score and bryophyte cover (p = 0.023), but no significant relationship between cover and the Instability Index. These results suggest ed that our method is a useful and potentially powerful tool to accurately quantify substrate movement in steep headwater streams.