Groundwater exchanges near a channelized versus unmodified stream mouth discharging to a subalpine lake

Abstract

The terminus of a stream flowing into a larger river, pond, lake, or reservoir is referred to as the stream‐mouth reach or simply the stream mouth. The terminus is often characterized by rapidly changing thermal and hydraulic conditions that result in abrupt shifts in surface water/groundwater (sw/gw) exchange patterns, creating the potential for unique biogeochemical processes and ecosystems. Worldwide shoreline development is changing stream‐lake interfaces through channelization of stream mouths, i.e., channel straightening and bank stabilization to prevent natural meandering at the shoreline. In the central Sierra Nevada (USA), Lake Tahoe's shoreline has an abundance of both “unmodified” (i.e., not engineered though potentially impacted by broader watershed engineering) and channelized stream mouths. Two representative stream mouths along the lake's north shore, one channelized and one unmodified, were selected to compare and contrast water and heat exchanges. Hydraulic and thermal properties were monitored during separate campaigns in September 2012 and 2013 and sw/gw exchanges were estimated within the stream mouth‐shoreline continuum. Heat‐flow and water‐flow patterns indicated clear differences in the channelized versus the unmodified stream mouth. For the channelized stream mouth, relatively modulated, cool‐temperature, low‐velocity longitudinal streambed flows discharged offshore beneath warmer buoyant lakeshore water. In contrast, a seasonal barrier bar formed across the unmodified stream mouth, creating higher‐velocity subsurface flow paths and higher diurnal temperature variations relative to shoreline water. As a consequence, channelization altered sw/gw exchanges potentially altering biogeochemical processing and ecological systems in and near the stream mouth.

Nutrient processes at the stream‐lake interface for a channelized versus unmodified stream mouth

“…A recent study focusing on the immediate region where streams discharge to freshwater lakes reveals unique physical and hydrologic characteristics expected to affect nutrient cycling [ Constantz et al ., ]. These regions, referred to herein as the stream‐lake interface (SLI), had not generally been distinguished from the broader stream or lake environment.…”

“…Constantz et al . [] identified two distinct types of SLIs, one that has been channelized to confine and focus flow and one that naturally meanders and periodically forms a barrier beach and backwater pond adjacent to the lakeshore. Direct and indirect hydraulic and thermal estimates of seepage fluxes indicated profound impacts of channelization on SW‐GW exchanges at the SLI.…”

“…Water quality parameters, including concentrations of DIN and DIP in surface water and shallow subsurface water, are analyzed with respect to mean residence times to quantify the conceptual hydraulic models of SLIs developed by Constantz et al . [] and explore their implications for water quality. Spatial relationships of nutrient concentrations, DO, and DOC are compared to mean residence times in two types of SLIs, channelized and natural.…”

Active heat pulse sensing of 3-D-flow fields in streambeds

“…where T is temperature ( • C) and D t is the thermal dispersion coefficient given as (de Marsily, 1986)…”

“…Series of vertical temperature profile sticks installed along transects have also be used in other studies to examine 2-D flow fields in the streambed (Constantz et al, 2013(Constantz et al, , 2016Shanafield et al, 2010). When using ambient temperature fluctuations and one-dimensional heat transport models, several days of data are required to estimate the vertical flux.…”