CAN RAPID ASSESSMENT PROTOCOLS BE USED TO JUDGE SEDIMENT IMPAIRMENT IN GRAVEL-BED STREAMS? A COMMENTARY

摘要

Land management agencies commonly use rapid assessments to evaluate the impairment of gravel-bed streams by sediment inputs from anthropogenic sources. We question whether rapid assessment can be used to reliably judge sediment impairment at a site or in a region. Beyond the challenges of repeatable and accurate sampling, we argue that a single metric or protocol is unlikely to reveal causative relations because channel condition can result from multiple pathways, processes, and background controls. To address these concerns, a contextual analysis is needed to link affected resources, causal factors, and site history to reliably identify human influences. Contextual analysis is equivalent in principle to cumulative effects and watershed analyses and has a rich history, but has gradually been replaced by rapid assessment methods. Although the approaches differ, rapid assessment and contextual analysis are complementary and can be implemented in a two-tiered approach in which rapid assessment provides a coarse (first-tier) analysis to identify sites that deserve deeper contextual assessment (second-tier). Contextual analysis is particularly appropriate for site-specific studies that should be tailored to local conditions. A balance between rapid assessment and contextual analysis is needed to provide the most effective information for management decisions.