River-Aquifer Interactions, Geologic Heterogeneity, and River Management

Managing rivers and their underlying aquifers for minimum flows, riparian habitat or aquifer recharge requires an understanding of the spatial patterns and temporal dynamics of river-aquifer exchange. Results are presented from investigations of the effects of geologic heterogeneity on river-aquifer exchange, minimum river flows and water availability in the riparian corridor for a typical alluvial fan system in the western USA. River-aquifer interactions were simulated on a regional-scale (~50km) and for a river reach (~2000m) using numerical codes for saturated and variably saturated flow. Geologic heterogeneity of the alluvial fan system was characterized with a geostatistical approach, based on transition probabilities and Markov Chains. Different hydrofacies models for a 50 km segment and a 2 km reach were created from sequential indicator simulations. Variably saturated flow between the river and the deep regional water table were explicitly simulated in both models. Groundwater levels, river flows, sediment saturation and temperature data were used to calibrate the models. The regional simulations showed that different spatial arrangements of hydrofacies have significant effects on minimum river flows with implications for salmon migration. Although total annual seepage volumes were relatively insensitive to geologic heterogeneity spatial and temporal variability of seepage was large between the different heterogeneous models. Local reconnections developed seasonally in some models and the period with sufficient flows for salmon fall-migration varied by up to 13 days between the models. The reach-scale simulations demonstrated that perched zones, which form between the river and the regional water table, can be important in supporting river base flows and riparian vegetation. Connected pathways between the river channel and the riparian corridor, which could be characterized with the temperature data, may sustain phreatophytes even when the regional water table is far below the river channel. These results elucidate some important effects of geologic heterogeneity on river-aquifer interactions, which could be crucial for the management of alluvial river systems.