HYPORHEIC EXCHANGE AS A CONTROL ON POPULATIONS OF MUSSELS IN THE ALLEGHENY RIVER, PENNSYLVANIA

Endangered and non-endangered mussels are heterogeneously distributed within the Allegheny River in northwestern Pennsylvania and hydrologic controls were suspected to influence mussel distribution. This study was designed to determine whether hyporheic exchange is related to distribution and density of mussel populations. Secondarily, this project tests the ability of seepage meters to function in high velocity riverbeds containing large substrate sizes (up to cobble and boulder). Sets of seepage meters installed within five river reaches, varying from high to low mussel density, recorded the amount of exchange across the sediment-water interface for one day periods during July, 2009. Simultaneously, temporary manometers driven to depths greater than 30 cm within the streambed, and adjacent to each seepage meter, displayed the pressure differential between the surface and hyporheic waters. Temperature loggers recorded thermal differences between the surface water and hyporheic waters 30 to 50 cm beneath the sediment-water interface. Pebble counts identified mean sediment size of each reach. Areas of high mussel density contained mean hyporheic exchange rates from 25 to 105 cm/d for both upward and downward flows. Conversely, areas of low mussel density showed mean exchange rates of 54 to 209 cm/d for upward and downward flow. R² for this inverse relation between seepage flux and mussel density was 0.85. Median sediment sizes were 49, 54, and 57 for high, moderate, and low density sites respectively. Thermal probes did not record a relation between temperature gradient and mussel density (R² = 0.13). Findings suggest that there is an inverse relationship between hyporheic exchange and mussel population density, while thermal exchange and substrate size appears not to correlate. Additionally, this study demonstrates the ability for seepage meters to record exchange rates in beds containing large sediment clasts (D₈₄= 203 mm). Overall, these findings conclude that hyporheic exchange rates are a factor related to creating a healthy ecosystem required for repopulation of endangered and threatened mussels.