2018 Water Quality Monitoring Report

Since 2009, West Multnomah Soil & Water Conservation District’s (WMSWCD) has been monitoring water quality in the rural part of western Multnomah County on perennial streams that flow directly into the Multnomah Channel. This report focuses on sites where we performed continuous temperature monitoring between May 18th and October 10th, 2018, including Crabapple Creek, Miller Creek, Sheltered Nook, and McCarthy Creek. (To be consistent with past monitoring years, data presented in our report cover the period of May 22nd through October 7th, 2018.)

This summer 2019 will be the 10-year anniversary of the first sampling on McCarthy Creek, and we’re excited that we are getting close to having enough data to see trends. We’re watching for increases in-stream temperature that affect water quality and organisms that live in or depend on the stream for part of their life cycle. This is particularly true for salmon and other fish species that depend on cold water habitats. Warm water also contributes to the growth of algae. Tracking in-stream temperature trends informs future conservation planning for waterways and helps us assess the potential success of efforts to shade and cool streams with streamside tree canopy cover.

2018 weather summary:

The summer of 2018 was another hot and dry one, marking the second such summer in a row. Air temperature was above the average for the study period, including 29 days over 90 degrees Fahrenheit (U.S. Climate Data, 2018; Weather Underground, 2018).

Precipitation was below normal for the study period. Only 22 out of the 139 testing days reported a precipitation event, totaling only 2.89 inches of rainfall throughout the study period (Weather Underground, 2018).

Conclusion:

The warm air temperature and lack of precipitation from just prior to May through October of 2018 likely resulted in the higher water temperature we measured in McCarthy and Crabapple Creeks. Changes in water temperature during the sampling season are directly correlated to precipitation events; the water temperature plummets at times when there is precipitation to cool the stream down (Figure 1).

Data continues to point towards relatively cool water in the headwaters for McCarthy, which has abundant forest cover and steep slopes that keep the water moving quickly. Conversely, in the mid-section of McCarthy Creek we see significant warming, likely from inadequate forest cover and riparian vegetation on both the mainstem creek and tributaries. WMSWCD has performed riparian restoration work on this middle stretch of creek to provide shade and habitat, and of all the sections of the creek that we are monitoring, it is the only section that shows temperatures lower than the average temperatures measured to date. Measured temperatures in the lower sections of McCarthy Creek are also high, likely due to a change in stream character with slower flowing water with more pools where water surface temperature tends to heat up.

For more detailed findings, download the full report. For more information on the water quality monitoring program, methods used, and the watersheds in the study area, please visit our Water Quality Monitoring page.