Hydrologic reconnaissance of the Chilkat River basin, southeast Alaska, with special reference to the bald eagle critical habitat at the Tsirku River alluvial fan

Abstract

The Chilkat River basin in southeast Alaska is characterized by rugged, highly dissected mountains with steep-gradient streams, braided rivers in broad alluvium-filled valleys, and numerous glaciers. A wide seasonal range in temperature and strong orographic effects cause variations in the amount and distribution of precipitation, and thus in the resulting runoff and streamflow. Streamflow is lowest in winter, when precipitation at higher altitudes is stored as snow, and greatest in summer, when melting snow and glacier ice augment flow. Ground-water seeps and springs flowing from alluvial fans contribute to streamflow year round. A ground-water discharge zone of particular interest is that along the toe of the Tsirku River alluvial fan, 20 miles north of Haines. During winter, the relatively warm (4 to 6 degrees Celsius) ground water maintains open leads in a reach of the Chilkat River downstream from the fan. This ice-free reach provides favorable spawning habitat for a late run of chum and coho (silver) salmon, which in turn attracts the world's largest concentration of bald eagles (more than 3,000 birds). The principal source of recharge to the ground-water system in the fan is loss of water through the beds of the many distributary channels across the fan surface. Calculation of a water budget for the system for the period October 1982 through May 1983 indicates that ground-water discharge at the toe of the fan averages 640 cubic feet per second. Surface and ground waters are chemically similar, calcium bicarbonate types. All stream samples had dissolved-solids concentrations of less than 115 milligrams per liter; values for ground water were slightly greater. During high summer flows, the suspended-sediment concentrations of the glacially fed Chilkat, Tsirku, and Klehini Rivers ranged from 361 to 1,530 milligrams per liter (6,360 to 22,300 tons per day).