Evaluating long-term effects of destructive flooding on in-stream riparian characteristics and macroinvertebrate abundance in low order streams

Background/Question/Methods

In August of 2009 a flashflood scoured an assemblage of sixteen 1^st – 3^rdorder headwater streams surrounding Zoar Valley Canyon in western New York State USA. Environmental and biotic features of these streams were quantitated in 2006 and reported in the peer reviewed literature. The objective of this study is to determine long-term disturbance effects on environmental characteristics and biotic composition of impacted streams. This aspect of stream ecology has been relatively unexamined in mesic environments, especially where quantitative pre-disturbance data are available. Biotic and environmental assessments from 2010 and onward will mirror those from 2006, with biota collected from riffle/cobble segments by Surber net, and environmental/habitat variables quantified by maps and aerial photos, ground-truthing, and a widely-used Qualitative Habitat Evaluation Index (QHEI) that assesses in-stream and riparian characteristics.

Results/Conclusions

QHEI scores first calculated in 2006 dropped markedly, with 11 of 16 of the streams included in this study reduced at least one habitat grade as of 2010 – 2011 (e.g. from ”excellent” to “good”, or “good” to “fair”). Larger 2^nd- and 3^rd-order streams suffered the most, while in contrast, half of the 1^st- order streams (5 of 10) maintained their habitat grade. Although post-flood macroinvertebrate abundance in 2010 was still nearly two-thirds that in 2006 (surprisingly high), it was extremely patchy. Among-replicate (i.e. within-stream) coefficients of variance were consistently >100%; twice those in 2006. Declines were observed in a number of insect orders, including the mayflies, with the disturbance sensitive families Heptageniidea, Ephemerellidae, Ephemeridae, and Baetidae (all well represented in 2006) decreasing to near zero by 2010. In addition, the Chironomidae, Tipulidae, and Ceratopogonidae (all Dipterans) declined markedly from their abundance in 2006. Interestingly, these taxa tend to be most associated with fine organic sediments and periphyton, which may have been lost with the flood. Other taxa, however, remained abundant post-flood, including Leptophlebiidae mayflies, Hydropsychidae caddisflies, and Leuctridae stoneflies. Somewhat surprisingly, some taxa actually increased, including Ameletidae mayflies, Nemouridae stoneflies, and Philopotamidae caddisflies.