A lake located in a groundwater flow system in which all groundwater flow is into the lake is an effluent or gaining lake (Figures 32a and 33). The lake surface is an expression of the water table. In this setting, flow discharging to the lake causes the lake level to rise unless it is balanced by loss of water from the lake by way of direct evaporation, evapotranspiration or surface-water outflow. Water levels in the lake adjust in response to changes in the lake water budget.
Figure 33 – Cross section and map views of effluent (gaining) lake exchange. Equipotential lines and relative head values are shown in black. Groundwater flow is in the direction indicated by blue arrows. Aquifer conditions are assumed to be isotropic and homogeneous. Monitoring wells are open at the bottom. a) Cross sectional representation showing an upward groundwater gradient and groundwater discharging to the lake. Lake stage is shown as a water level on the vertical rectangle. b) A map view showing equipotential lines and groundwater flow converging at the lake. c) A map showing an effluent lake that has a stream discharge. Some groundwater may flow from the lake to the stream under these conditions (Woessner, 2020).
