Dynamically fragmented stream networks: field observations and physical drivers

The flowing portion of river networks experiences never-ending event-based and seasonal expansion/retraction cycles, which mirror the unsteady nature of the climatic forcing.  These "temporary" rivers constitute a major fraction of the global river network, and are found across a variety of settings, especially in the headwaters. A dominant feature of these channel network dynamics is represented by the fact that the active streams are dynamically fragmented, i.e. they do not simply expand upstream when the catchment wets up, to dry down in the downstream direction during the recessions. Instead, the wetting/drying processes frequently happen in complicated spatio-temporal patterns, either activating disconnected reaches first, that will only eventually get connected to the main channel, or generating disconnections by drying out segments in the middle of the river network before the flow heads start retracting. This contribution analyzes the spatial patterns of local persistency along river networks (i.e. the fraction of time for which flowing water can be observed at each location, which is related to the wetting/drying order of the different reaches) combining field data on the spatiotemporal evolution of flowing channels and theoretical analyses, with the aim of elucidating the physical drivers of stream connectivity and disconnectivity in temporary streams. The analysis reveals that river fragmentation is related to the spatial heterogeneity of subsurface properties other than the contributing area (e.g. slope, local width, permeability). The proposed framework provides a clue for analyzing the impact of the spatial and temporal heterogeneity of streamflow presence for a variety of morphologic and biogeochemical processes (sediment transport, ecological dispersion and stream metabolism).