Evaluating the Conservation Potential of Tributaries for Native Fishes in the Upper Colorado River Basin

We explored the conservation potential of tributaries in the upper Colorado River basin by modeling native fish species richness as a function of river discharge, temperature, barrier-free length, and distance to nearest free-flowing main-stem section. We investigated a historic period prior to large-scale water development and a contemporary period. In the historic period, species richness was log-linearly correlated to variables capturing flow magnitude, particularly mean annual discharge. In the contemporary period, the log-linear relationship between discharge and species richness was still evident but weaker. Tributaries with lower average temperature and separated from free-flowing main-stem sections often had fewer native species compared to tributaries with similar discharge but with warmer temperature and directly connected to free-flowing main stems. Thus, tributaries containing only a small proportion of main-stem discharge, especially those at lower elevations with warmer temperatures and connected to free-flowing main stems, can support a relatively high species richness. Tributaries can help maintain viable populations by providing ecological processes disrupted on large regulated rivers, such as natural flow and temperature regimes, and may present unique conservation opportunities. Efforts to improve fish passage, secure environmental flows, and restore habitat in these tributaries could greatly contribute to conservation of native fish richness throughout the watershed.