T. David Ritter | Search USGS

USGS2025

Water temperature regimes and thermal drivers in semi-natural and flow-regulated rivers of the northern Great Plains

Rivers of the northern Great Plains have lacked long-term, continuous water temperature assessments, and there is limited information on thermal regimes of these systems and factors driving water temperature. We collected and assembled 2001–2022 water temperature data from 18 sites on four reaches of three rivers that differ in anthropogenic impacts: semi-natural Yellowstone River (YR), flow-impacted Milk River (MK), 351-km of the Missouri River affected by hypolimnetic releases from Fort Peck Dam (FPD), and the semi-natural Missouri River (MR3093) upstream from FPD. Objectives were to: (1) compare May–September mean daily water temperature ( T w ), day of year of maximum water temperature ( T maxdoy ), and maximum water temperature ( T wmax ) among reaches, (2) evaluate air temperature ( T a ), river discharge ( Q w ), and dam-release water temperature ( T wdam ) as T w drivers, and (3) model longitudinal recovery of T w downstream from FPD. Mean T w and T wmax were greatest at the YR, MR3093 and MK sites, and significantly less through 291-km downstream from FPD. T maxdoy at initial sites downstream from FPD was delayed 43–69 days relative to the semi-natural reach upstream from FPD. T a was the primary correlate of T w for the semi-natural sites; whereas, T wdam and T a varied inversely as primary drivers for sites downstream from FPD. Thermal recovery from hypolimnetic releases was incomplete 291-km downstream from FPD and warming persisted 351-km downstream. Results quantify the varied water temperature regimes of rivers in the northern Great Plains and improve understanding of controls affecting T w among reaches. Water temperature attributes of semi-natural reaches could be used as restoration targets for 300-km of Missouri River presently impacted by hypolimnetic releases.

Montana, North Dakota↗

USGS2024

Spatial and temporal variability of movements among sympatric salmonids in an unfragmented inland watershed

Objective Our aim was to determine the movement patterns of three abundant salmonids—Brown Trout Salmo trutta , Mountain Whitefish Prosopium williamsoni , and Rainbow Trout Oncorhynchus mykiss —in the Smith River watershed of Montana. Methods We tagged 7172 fish with passive integrated transponder (PIT) tags, monitored their movements past 15 stationary PIT arrays over 4 years, and located tagged fish between arrays by conducting mobile surveys. Result Movement patterns varied seasonally, among species, and among locations. Movement was greatest in the middle portion of the watershed, which included a pristine main‐stem canyon and lower reaches of major tributaries. Fish rarely left the canyon, but movement into the canyon from other regions was common. Mountain Whitefish were most likely to move, and Brown Trout were least likely to move. Most fish travelled less than 10 km, but some fish travelled over 100 km. Distinct movement patterns were not evident; rather, a continuous spectrum of movement behaviors was apparent. Movements by Mountain Whitefish and Rainbow Trout increased during their spawning periods. Movements peaked when mean daily water temperatures were between 11.3 and 17.1°C. Conclusion Movements were diverse and probably contributed to metapopulation dynamics, population resiliency, and species diversity. Fish movements along stream networks connect populations across diverse landscapes, and therefore, protecting and restoring stream connectivity along inland streams such as the Smith River is critical to maintaining productive fish assemblages.

Montana↗