Hydrologic Connectivity and Secondary Channel Habitat Properties of the Lower Mississippi River, a Large, Meandering River in the Southern <scp>USA</scp>

ABSTRACT In large, meandering rivers, much of the instream flow veers around islands through secondary channels (SCs), which often comprise a substantial proportion of a river's longitudinal path. Due to their dissimilar hydrological conditions, within and among themselves and to the main channel (MC), SCs augment habitat diversity and biogeochemical potential of the river system. In this study, we investigated the effects of the flow regime on habitat properties of the MC and SCs of the Lower Mississippi River (LMR). Over 2 years, we sampled the MC and 14 SCs in June at high water (when all SCs were hydrologically connected to the MC and flowing) and in October–November at low water (when most had disconnected and were not flowing but retained water). We compared three connectivity categories (MC, connected SC, disconnected SC) for a suite of physical and chemical properties in the water column and sediments. For multiple properties, we found statistically significant differences as functions of connectivity, sampling period, and their interaction. In high water (all SCs connected), the SCs were most similar to the MC but still, on average, 43% lower in water velocity, 142% greater in sediment % Carbon, and 68% higher in sediment % organic matter. In low water, disconnected SCs stratified, with indications of the lower water column becoming chemically reducing. The three most important environmental parameters correlated with SC macroinvertebrate assemblage structure were flow velocity, sediment %organic matter, and sediment particle size. While we examined only a small subset of the many SCs along the LMR, this is the most complete study to date of their physiochemical habitat properties relative to connectivity and time.