Key stream/sediment exchanges of water and heat near stream mouths

Abstract:

The section of stream discharging to a lake or other surface-water body is referred to as the stream mouth, a stream reach with rapidly changing hydrologic conditions, leading to unique aquatic and benthic ecology, as well as a visibly active fishery habitat. Of environmental significance, bridges, control structures, channelization and foot traffic are common near stream mouths, warranting comparisons of natural and channelized stream mouths. The present work completes the first investigation focusing specifically on the hydrology of surface-water/sediment exchanges at stream-mouth reaches discharging to lakes and compares these exchanges to those measured along the nearby shoreline in both a qualitative and quantitative manner. Heat and water exchanges for two common types of stream mouths (a natural stream with a summer barrier bar and a channelized stream mouth) are compared with comparable exchanges along the nearby shoreline on the north shore of Lake Tahoe located in the Central Sierra Nevada Mountain Range (CA/NV, US). The study site was selected partially due the abundance of streams discharging into the lake of both a natural and channelized nature (~30 small streams with a large number of both types of stream mouths). Heat and water exchanges were both qualitatively and quantitatively distinct for the three types of hydrologic settings, with (1) cool, low velocity, longitudinal (hyporheic) flowpaths observed below the channelized stream mouth, discharging beneath the warmer, more buoyant lakeshore water, (2) the nearby shoreline receiving relatively warm, higher velocity discharge and (3) for the natural stream mouth, there was strong diurnal temperature pattern in groundwater discharging through the seasonal barrier beach to the lake. Impacts of strong 2013 wave action on exchanges were also distinct for the three settings, with (1) channelization allowing waves to extend well upstream, (2) a lesser invasive impact in the shoreline swash zone exchanges and (3) the least spatial impact on the natural stream with its seasonal barrier beach shielding the mouth.