An important issue surrounding biomonitoring in large rivers is the minimum sampling effort required to collect an adequate number of fish for accurate and precise determinations of biotic condition. During the summer of 2002, we sampled 15 randomly selected large‐river sites in southern Idaho to evaluate the effects of sampling effort on an index of biotic integrity (IBI). Boat electrofishing was used to collect sample populations of fish in river reaches representing 40 and 100 times the mean channel width (MCW; wetted channel) at base flow. Minimum sampling effort was assessed by comparing the relation between reach length sampled and change in IBI score. Thirty‐two species of fish in the families Catostomidae, Centrarchidae, Cottidae, Cyprinidae, Ictaluridae, Percidae, and Salmonidae were collected. Of these, 12 alien species were collected at 80% (12 of 15) of the sample sites; alien species represented about 38% of all species (N = 32) collected during the study. A total of 60% (9 of 15) of the sample sites had poor IBI scores. A minimum reach length of about 36 times MCW was determined to be sufficient for collecting an adequate number of fish for estimating biotic condition based on an IBI score. For most sites, this equates to collecting 275 fish at a site. Results may be applicable to other semiarid, fifth‐order through seventh‐order rivers sampled during summer low‐flow conditions.

No abstract

The 150‐kilometer middle reach of the Snake River (middle Snake) in south‐central Idaho receives large quantities of water from springs discharging along the north side of the river from the regional Snake River Plain aquifer. Water‐quality samples collected from nine north‐side springs in April 1994 indicated that springs in the upstream part of the reach had larger concentrations of dissolved solids, dissolved nitrate, total nitrogen, tritium, and heavy isotopes of hydrogen and oxygen than to springs in the downstream part of the reach. Because the spring chemistry varies in the reach, discharge from the springs resulted in a degradation in water quality in some parts of the middle Snake and improvements in water quality in other parts. Depending on the annual discharge in the Snake River, the contribution from the north‐side springs represented 33 to 66 percent of the discharge, 32 to 57 percent of the dissolved solids, 26 to 50 percent of the total nitrogen, and 7 to 14 percent of the total phosphorus transported annually from the middle Snake. Synoptic sampling showed that the north‐side springs contributed 84 percent of the discharge and 35, 40, and 10 percent of the dissolved solids, total nitrogen, and total phosphorus load, respectively, to the Snake River during the peak of the irrigation season in 1994.

Daily mean discharge exceeded 20 percent of the time during a specified month Q.50 Daily mean discharge exceeded 50 percent of the time during a specified month (same as median discharge) Q.80 Daily mean discharge exceeded 80 percent of the time during a specified month