Managing sources, sinks and transport of natural contaminants in regulated rivers: a case study in the Murrumbidgee River catchment, NSW

Environmental flows are assuming a central role in the sustainable management of Australian rivers. These flows are intended to recreate a flow regime and habitats which existed before the onset of flow regulation. Such flows, however, are now being implemented in a highly modified landscape, resulting in drastic changes to contaminant cycles which are now highly dependent on regulated, in-channel flows. This study examines the dynamics of contaminants (nutrients and salts) in each of three major riverine habitats; water column, sediments and biofilms, to identify sub-catchments with the highest potential for supplying salts to the main stem of the Murrumbidgee River, south-eastern Australia. Concentrations of contaminants in the sediments and biofilms were two orders of magnitude higher than in the water column in all sites, highlighting their importance as a sink for salts during low flows, and a potential source during in-stream freshes. By managing the timing and magnitude of regulated flow releases, environmental flows could be used to either enhance in-stream productivity by scouring nutrients and salts from in-channel habitats, diluting high loadings of unwanted contaminants (salts) transported from degraded sub-catchments, or reduce nutrient availability for downstream foodwebs by diluting nutrient rich floodwaters from catchment scale runoff events.