Longitudinal river ecohydrology: flow variation down the lengths of alluvial rivers

Longitudinal flow variation is an emerging field of study in river ecohydrology. Longitudinal changes in the frequencies, magnitudes, durations and timing of floods, low-flows and intermittence create a dynamic environment for flow-dependent species and ecological processes. Analyses of flow variation and flow-ecosystem relationships in the longitudinal dimension require synoptic flow time-series at multiple sites along a river. Complex channel geomorphology and a scarcity of rivers equipped with multiple flow gauges have slowed progress in longitudinal ecohydrology. The empirical longitudinal flow model (ELFMOD) is a new statistical tool for estimating flows at multiple points along rivers; which circumvents the requirement for multiple gauges. In this study, we used ELFMOD to explore longitudinal flow variation in four alluvial rivers from different geological terrains and climate zones: the Albarine (France), Methow (USA) and Selwyn and Orari (New Zealand). Differences among rivers in longitudinal and temporal flow variation were evident on date x location flow matrices and longitudinal profiles. There were notable differences among longitudinal gradients in flow magnitude and flow percentiles, and in the recurrence, extent, and rate of expansion and contraction of dry river reaches. There were also large differences in longitudinal flow-permanence patterns. Temporal variation in the lengths of dry channel and numbers of dry reaches was predominately seasonal in the Albarine and Methow, and interannual in the Orari and Selwyn. Broad differences among the rivers in longitudinal flow patterns correspond to differences in the configuration of hydrogeomorphic discontinuities such as tributary confluences, channel divergences and convergences, and lithological contacts. Copyright (C) 2010 John Wiley & Sons, Ltd.