LAUSR

High turbidity in reservoirs may negatively affect aquatic ecology and water infrastructure. Although many reservoirs are turbid, the origin and intra- and interannual variability of turbidity is not well understood. To examine the cause and variability of turbidity, we measured the concentration, particle size, organic content and mineralogy of suspended and benthic sediment samples from Lake Dalrymple, a large reservoir in northern dry tropical Queensland. Samples were collected during the dry season (May–November) in 2004, 2005, 2006 and 2011. Water in the reservoir is turbid (>45 nephelometric turbidity units (NTU)) throughout the year, predominantly due to colloidal particles (<0.9μm) composed of clay and mica minerals. Benthic sediments range from 1 to 30μm in size, suggesting that resuspension of lake bottom sediments has little effect on turbidity. Dry season turbidity levels are strongly affected by the magnitude and duration of inflow waters from the preceding wet season, reflecting the limited availability of suspended particles delivered from the upstream catchment. Elevated turbidity in the reservoir and downstream irrigation areas during the dry season is now a common feature and must be incorporated into all agricultural and wetland management plans.