Measures of Physical Heterogeneity in Appraisal of Geomorphic River Condition for Urban Streams: Twin Streams Catchment, Auckland, New Zealand

Abstract

This investigation created and tested a template to rapidly assess geomorphic river condition in urban settings. This extension to the River Styles Framework® (Brierley and Fryirs, 2005) entailed mapping the heterogeneity in bed material, habitat, and flow characteristics for different types of rivers, integrating parameters from geomorphology, ecology, and hydrology. Analysis was carried out at 27 sites in the Twin Streams catchment in West Auckland, New Zealand. The method successfully recorded the extent of degradation of physical structure following European settlement of the catchment. With the exception of one subcatchment, streams were found to be largely intact in the headwaters. Many of these headwater streams were found to be of exceptional quality, with high physical heterogeneity. Geomorphic condition is more degraded in downstream areas. Fine-grained sediment has smothered stream courses in the lower half of the catchment, covering bed material and creating homogenous structure and flow, decreasing quality of habitat for biota. In this more urbanized area, with more stormwater drains, riparian vegetation is limited and of poor to moderate quality. Understanding of geomorphic responses to human disturbance is critical in the design and implementation of effective management strategies that seek to improve the ecological condition of urban streams.