SPRINGFLOW EFFECTS ON CHEMICAL LOADS IN THE SNAKE RIVER, SOUTH-CENTRAL IDAHO

The 150-kilometer middle reach of the Snake River (middle Snake) in so uth-central Idaho receives large quantities of water from springs disc harging along the north side of the river from the regional Snake Rive r Plain aquifer. Water-quality samples collected from nine north-side springs in April 1994 indicated that springs in the upstream part of t he reach had larger concentrations of dissolved solids, dissolved nitr ate, total nitrogen, tritium, and heavy isotopes of hydrogen and oxyge n than to springs in the downstream part of the reach. Because the spr ing chemistry varies in the reach, discharge from the springs resulted in a degradation in water quality in some parts of the middle Snake a nd improvements in water quality in other parts. Depending on the annu al discharge in the Snake River, the contribution from the north-side springs represented 33 to 66 percent of the discharge, 32 to 57 percen t of the dissolved solids, 26 to 50 percent of the total nitrogen, and 7 to 14 percent of the total, phosphorus transported annually from th e middle Snake. Synoptic sampling showed that the north-side springs c ontributed 84 percent of the discharge and 35, 40, and 10 percent of t he dissolved solids, total nitrogen, and total phosphorus load, respec tively, to the Snake River during the peak of the irrigation season in 1994.