Comment: A Comparison of Visual and Measurement-Based Techniques for Quantifying Cobble Embeddedness and Fine-Sediment Levels in Salmonid-Bearing Streams

摘要

McHugh and Budy (2005) address fundamental measurement questions regarding the accuracy of visual estimates of percent fines and levels of cobble embeddedness. They compare visual estimates with other techniques that are based on physical measurements. The results for percent fines indicate a relatively good precision about their regression line and suggest that visually based estimates can be relied upon for management purposes, given an appropriate observer calibration. Nonetheless, comments made by Graham et al. (2005) concerning the use of pebble counts for determining fines concentrations apply. In the case of embeddedness, McHugh and Budy (2005) obtained inconsistent results for the visual and alternative measurement techniques. The inconsistency was attributed as most likely resulting from methodological differences in which the "visual" embedded-ness values reflected an area-based quantity and the "measured" values reflected a depth-based quantity, but further details were not given. An analytic basis is given below as to why these two fundamentally different attributes cannot be compared directly and how they result in different quantities.