Water-body size is one of the most important factors affecting the structure and function of aquatic ecosystems. The categorical variable, Strahler stream order, is commonly used as a surrogate for stream size, perhaps because stream size is a multidimensional attribute that defies simple definition. Some stream-size attributes, including continuous variables, such as catchment area, distance to source, and model estimates of discharge also are available as geographic information system (GIS)-derived or modeled variables. These GIS measures are commonly used by stream ecologists along with field-derived attributes, such as discharge, stream cross-sectional area, width, and depth, which are more direct measures of stream size as experienced by aquatic organisms. Our objective was to quantify how well some commonly used stream-size attributes are predicted from Strahler order in the US as a whole and within major ecoregions and hydrologic landscape regions. We based our analysis on field-channel-survey and digital-stream-trace data (NHD-Plus) from 2162 US stream sites, ranging from 1st to 8th order (at 1∶100,000 scale). Strahler order provided a surprisingly useful approximation of the ranges of catchment size, distance to source, modeled mean annual discharge, and field-based low-flow and bankfull channel dimensions for most streams within a given Strahler order. However, even within geoclimatically and ecologically similar regions, site-specific predictions of stream size from Strahler order can have large errors. Correlations between Strahler order and the size measures considered here varied widely (r = 0.48–0.91). Within individual Strahler orders, the alternative size measures varied by 5 and 4 orders of magnitude at national and regional scales, respectively. The same size-measure value could occur in 1 to 7 different stream orders at the national scale and in some regions, with generally good agreement in mountains and poor agreement in plains. Therefore, we conclude that Strahler order is useful for relating information about stream size, but that researchers should base analyses on multiple, continuous measures of stream size and should communicate stream-size results or associations based on the size-related measurements. Two characteristics of Strahler order make it useful for selecting sites across the range of stream sizes encountered in regional and national surveys, as long as limitations are explicitly recognized. First, the number of Strahler orders is limited. Second, Strahler order is easy to extract from stream networks constructed from digital elevation data and from national hydrographic data sets.
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