Freshwater temperature trend in the intensively monitored watershed of the middle fork John Day River, Oregon

Abstract

Stream restoration is a rapidly developing field, and effectiveness monitoring is critical for informing restoration design and identifying adaptive management opportunities. Stream temperature is a driver of many ecological processes in aquatic environments and has been identified as a limiting factor for juvenile salmonids in many systems in the Pacific Northwest. Therefore, my study investigated temperature trends at 86 temperature monitoring locations in the Middle Fork John Day River, Oregon – a watershed which has been the subject of intense restoration efforts over the last 15 years. I performed trend analysis for the months of July, August, and September, using response metrics of total degree hours and degree hours above the temperature threshold causing stress to juvenile salmonids. These two metrics were examined using both unadjusted values and by adjusting values for annual variation in streamflow and air temperature. Many sites did not exhibit significant trends during the period of record. Results for unadjusted temperature metrics were dominated by tributary locations, had a relatively even distribution between increasing and decreasing trends, and decreasing trends were generally not located in restoration reaches. Flow and air temperature adjusted metrics were more evenly distributed between mainstem and tributary locations, were mostly decreasing, and a greater proportion of trends were located in restoration reaches. The relatively small number of significant trends compared to the number of tests performed indicates that the system is generally temperature-stable. Tributary systems dominate temperature metrics that are not adjusted for air temperature and stream flow, and may be more sensitive to external influences and annual variation of external drivers. Lastly, the temperature-mitigating effects of restoration tend to emerge after accounting for stream flow and air temperature, suggesting that restoration efforts currently have less influence over stream temperature than fluctuations in annual climate. Some of the benefits of restoration may take additional time to be realized and continued monitoring will be necessary to capture long-term effects. Historic trends in stream flow and air temperature, as well as projections of future climate conditions, suggest that restoration effectiveness will need to increase to outpace the influence of background climate effects.