Impact of Hydrologic Alteration on Brazos River Pelagophilic Minnows

Abstract

AbstractThe lower Brazos River has been profoundly altered since the construction of multiple impoundments. The resulting river regulation poses problems for organisms adapted to a natural flow regime and is thought to play a major role in the imperilment of seven native minnows (Smalleye Shiner Notropis buccula, Sharpnose Shiner N. oxyrhynchus, Shoal Chub Macrhybopsis hyostoma, Plains Minnow Hybognathus placitus, Ghost Shiner N. buchanani, Silverband Shiner N. shumardi, and Chub Shiner N. potteri) that synchronize spawning during high flow pulses. Four of the species are presumably extirpated from the region, but their declines and losses differed across time. To address potential reasons for species' declines, we assessed pre‐ and postimpoundment changes in flow regime from five gauging stations across the lower Brazos River to identify river flow characteristics that depart from the natural preimpoundment flow regime. For the seven minnows, there was a strong relationship between the year they were last documented and their historical percentage occurrence in the lower river. Differences in flow duration curves and flow behavior were generally consistent with position in the drainage such that the greatest changes occurred in the highly fragmented upper reach of the study area. However, extensive hydrologic alteration was apparent at all five localities. The extirpated species in the lower Brazos River may have been more sensitive to altered flow regimes and accompanying geomorphic change or were population sinks dependent on recolonization or drift from upstream populations. The relationships between pelagic‐broadcast‐spawning minnows and the flow regime are complex, and nonflow variables, such as population fragmentation, thermal regimes, geomorphology, and larval habitat dynamics, should be considered in environmental flow assessments to aid our understanding of biodiversity loss in this group of species across the North American plains. Managing flows to support species that depend on natural flow regimes will require not only a better understanding of their reproductive life histories, but also the ecology and physiology of early life history stages.