Hydrologic Trends and Spatial Relationships of Stream Temperature and Discharge in Urbanizing Watersheds in the Portland Metropolitan Area of the Pacific Northwest

• Main Author: Brenneman, Emma Lee
• Format: Others
• Published: PDXScholar 2019
• Subjects: Geography
• Online Access: https://pdxscholar.library.pdx.edu/open_access_etds/5128; https://pdxscholar.library.pdx.edu/cgi/viewcontent.cgi?article=6203&context=open_access_etds

This study explores various relationships of streamflow and stream temperature over the Portland Metropolitan area in two urbanizing watersheds. Four stream temperature and discharge metrics were derived from USGS stream gauges in the Tualatin River and Johnson Creek watersheds and were analyzed for monotonic trends. Additionally, this study explored the sensitivity of stream temperature to air temperature and streamflow to assess where locations throughout the watershed may be more sensitive to these changes. Relationships among stream temperature, air temperature, and streamflow were assessed using linear and nonlinear bivariate regression for yearly values and summer months. Additionally, this study seeks to explain the spatial variations of thermal sensitivity throughout the Johnson Creek watershed using predictors derived using different weights at the contributing watershed scale and the buffer scale. Results indicate significant increasing trends in stream temperature metrics at various locations throughout the study area. Decreasing baseflow does not appear to coincide with increasing temperature metrics. Significant increasing trends in October and November are present in runoff ratio and TQmean. In both watersheds, air temperature appears to have a greater influence than streamflow on stream temperature, though the addition of discharge generally improves model fit. Increasing thermal sensitivity in Johnson Creek is related to increasing and decreasing standard deviation of slope, increasing mean slope, increasing open water and wetlands, less forest area, increasing standard deviation of NDVI, decreasing restoration area, increasing gray infrastructure density, and increasing upstream flow length. At most, ordinary least squares explained 30% of the variance in thermal sensitivity when only including stream temperature monitoring locations in the mainstem of the creek. Modelling tributary only stream temperature monitoring locations used a variety of watershed, buffer-scale, areal average and inverse distance weighted variables. The findings of this study highlight the importance of temporal scale and complex hydro-climatic influences along an urban-rural gradient in assessing patterns of discharge and temperature. These results have important implications for watershed managers, local agencies, and stakeholders who have worked to restore Johnson Creek and help to guide future water quality planning throughout the watershed.