Total mercury and methylmercury were determined by use of low (subnanogram per liter) level analytical methods in 225 representative water samples collected following ultraclean protocols at 25 Indiana monitoring stations in a statewide network, on a seasonal schedule, August 2004-September 2006. The highest unfiltered total mercury concentrations were at six monitoring stations - five that are downstream from urban and industrial wastewater discharges and that have upstream drainage areas more than 1,960 square miles and one that is downstream from active and abandoned mine lands and that has an upstream drainage area of 602 square miles. Total mercury concentrations in unfiltered samples ranged from 0.24 to 26.9 nanograms per liter (ng/L), with a median of 2.35 ng/L. The highest concentrations of total mercury, those in the 90th percentile and above, were more than 9.05 ng/L, and most were in samples collected during winter and spring 2006 during changing streamflow hydrograph conditions. Seasonal medians for unfiltered total mercury were highest during winter and spring. Instantaneous streamflow and turbidity at the time of sample collection also were highest in winter and spring and potentially indicate conditions for the most particulate mercury transport. Samples with the highest total mercury concentrations were from water that had the highest turbidity at the time of sample collection. Unfiltered total mercury concentrations were significantly lower in samples collected at five stations downstream from dams. Values for particulate total mercury and streamflow also were significantly lower at these five stations. Total mercury concentrations equaled or exceeded the 2007 Indiana chronic aquatic criterion of 12 ng/L in 5.8 percent of samples and at 10 monitoring stations. Most of the total mercury in these 13 samples was estimated to be particulate. Most of the samples with mercury concentrations that equaled or exceeded the 12 ng/L criterion were collected during winter and spring 2006 during changing streamflow hydrograph conditions and in streamflow that was high for 2004-2006. Methylmercury was detected in 83 percent of unfiltered samples; reported concentrations ranged from 0.04 to 0.57 ng/L, with a median of 0.09 ng/L. The highest concentrations of methylmercury, those in the 90th percentile and above, were more than 0.25 ng/L, and most were in samples collected during spring and summer. Methylation efficiency in most samples was less than 5.8 percent, but was as much as 24.6 percent. Seasonal medians for methylmercury were highest during spring and summer. Seasonal medians for water temperatures at the time of sample collection were highest during these seasons and potentially indicate conditions for the most formation of methylmercury. The low streamflow statistical category had the significantly highest methylation efficiency.