Drainage from the Critical Zone: Lithologic Controls on the Persistence and Spatial Extent of Wetted Channels during the Summer Dry Season

In seasonally dry environments, critical zone drainage provides base flow that sustains river ecosystems. The extent of wetted channels and magnitude of base flow throughout the network, however, are rarely documented, and no general theory currently exists enabling the prediction of these key ecosystem properties. We conducted channel surveys in early and late summers of 2012, 2014, and 2015 in four headwater drainage networks (2.8-17.0 km(2), in the Franciscan Formation of the Eel River (Northern California)), two of which are underlain by the Coastal Belt (argillite and inter-bedded sandstone) and two of which are underlain by the Central Belt (sheared argillaceous-matrix, meta-sedimentary melange). In all networks, stationary springs controlled the extent of flow. Though surveyed during a period of multiyear drought, the two adjacent Coastal Belt networks remained flowing throughout late-summer months, sustained by drainage from groundwater stored in thick weathered bedrock above fresh, impermeable bedrock. Flow magnitudes, however, decreased and surface flows became increasingly discontinuous, largely due to infiltration into thick gravel deposits on the channel bed. Only 23 km away, in the Central Belt melange, channel flow ceased early in the summer because the thin critical zone (typically <3 m) stored little water. All late-summer flowing water initiated from deep-rooted sandstone blocks and terminated a short distance downslope. Our findings suggest that lithology and critical zone development exert primary controls on wetted channel extent. Given similar annual precipitation, nearby watersheds can have dramatically different summer wetted channel networks that result in fundamentally different aquatic ecosystems.