Image above: The author, fully armored against biting insects, sets up an autosampler to collect stream samples..
Photo Credit: Erica Lamb.
Flow configuration is intrinsically complex in single channels; this complexity increases at the confluence of two streams, where active mixing is expected. Multiple studies have been conducted at the convergence of two channels in temperate climates, but none have been completed in the colder waters of the northern latitudes. While active mixing is anticipated at a confluence, it can be markedly reduced if suspended sediment concentrations and water temperatures differ between the converging channels. In cases of limited mixing, an interface between the flows can be expected. We present initial discharge, suspended sediment and water temperature measurements carried out during early spring and mid-summer at the confluence of two streams in northern Interior Alaska. The Malamute Fork flows into the Alatna River at 67° 01’ 54” N, 153° 16’ 07” W. The contributing watershed areas are 2091 and 6294 km2 for the Malamute Fork and Alatna River respectively. Discharge measurements ranged from 28 to 158 m3/s, suspended sediment concentrations ranged from 7 to 114 mg/l and near-surface water temperatures ranged from 5 to 14° C. Results indicate that the discharge distribution upstream of the confluence between the Malamute Fork and the Alatna River ranged from 40 to 48%, with the Alatna River having the larger contribution. The suspended sediment concentration ratio varied between 50 and 75%, with the Alatna carrying a larger suspended sediment load than the Malamute Fork. Differences in water temperature between the two channels were about 2° C, with sharp temperature gradients during spring measurements. In addition, vorticity plots at the confluence showed relatively low vorticity values along the water interface, indicating little mixing between the waters of different origin. The water interface can still be detected 2 km downstream of the confluence. We speculate that the roles of water temperature and sediment concentration differences in reducing mixing along the water interface are variable during the open water season. The magnitude of the contribution of water temperature and sediment concentration towards reducing mixing will be examined in the near future.
