The Selwyn River of New Zealand: a benchmark system for alluvial plain rivers

Abstract

AbstractMost of the world's alluvial plain rivers have undergone hydrological and geomorphical modifications due to water abstraction, dam and levee construction, gravel mining and other human activities. Some of these rivers function as benchmark systems for identifying and quantifying the ecological responses to hydrological and geomorphological changes. Benchmark systems are critical for understanding these responses, for predicting the effects of future changes, and for trialling restoration and mitigation measures. The Selwyn River of New Zealand is a benchmark system for undammed alluvial rivers that are under intense pressure for water abstraction, and are subject to large flow fluctuations.The Selwyn is a remarkably complex river, and increased understanding of this system will provide insight for understanding and managing other rivers in its class. Hydrological properties that characterize the Selwyn include strong surface water–groundwater interactions, contiguous ephemeral, intermittent, perennial‐losing and perennial‐gaining reaches and an expanding and contracting dry segment that persists for most of the year. The dry segment, in combination with broad spatial variation in aquifer structure and rainfall, cause the upstream (runoff‐fed) and downstream (groundwater‐fed) river sections to function very differently. These sections are also dissimilar in channel morphology; the upstream section has a braided planform, with mobile bars, and abundant islands and remnant channels, and the downstream section has a single, meandering channel, stable bars and no islands. As in many alluvial plain rivers, large floods drive reach‐scale channel evolution.This paper introduces a long‐term research program that is underway at the Selwyn River, and explores the hydrological and morphological dynamics that characterize the river. We focus on groundwater–surface water interactions, flow‐permanence patterns and flood‐dependent geomorphology. Hydrological and meteorological data are summarized in a conceptual model of relationships between prevailing weather systems, runoff, aquifer recharge and river flow. The physical template described in this paper governs ecological processes such as dispersal, succession and nutrient cycling. A conceptual model is proposed to organize predictions about dispersal in response to changes in hydrological connectivity. Copyright © 2007 John Wiley & Sons, Ltd.