Implications for Water Quality

Although the objective of this report is to describe the physical hydrology of the Johnson Creek basin in terms of how water moves in space and time, findings also relate to water quality. Stream temperature responds to air temperature and to groundwater discharge to the stream. These results will help watershed managers design and assess the effectiveness of measures to reduce summer stream temperature. A previous study indicated a positive relation of organochlorine pesticides to suspended sediment in the stream (Tanner and Lee, 2004). Although this study did not involve collection of sediment or contaminant data, analyses provide insight into high flows that typically entrain sediment (and potentially contaminants) in Johnson Creek. An understanding of controlling factors of stream temperature during low flow and runoff response during high flow may help in the decisions made to maintain and improve water quality in the basin.

Stream Temperature

Fluctuations in the temperature of Johnson Creek and tributaries are a result of human and natural causes. Low summer stream temperature supports more native fish life, whereas warm temperatures may be harmful to many aquatic organisms, raising concern regarding survival of native fish and other wildlife. A return to a stream temperature regime closer to the predevelopment condition would likely encourage a more native aquatic species composition. The effects of human modification in a stream basin generally cause increases in stream temperature compared to a predevelopment condition. Soil compaction decreases infiltration, which decreases groundwater discharge to the creek. Urban development may direct runoff to a combined sewer system, which also decreases groundwater discharge. Removal of vegetative cover and installation of ponds increases direct sunlight to the stream. Water withdrawals for irrigation decrease stream velocity and depth, increasing warming from the atmosphere.

The Total Maximum Daily Load (TMDL) allocation for temperature of Johnson Creek was set in 2006; it provided goals for reduction of summer stream temperature by increasing nearstream shading (Oregon Department of Environmental Quality, 2006). Because of the large geographic scale of the TMDL and lack of information on groundwater discharge to Johnson Creek, the effect of groundwater discharge on stream temperature was not fully explored. This study reports summer stream temperature decreases in areas of groundwater discharge to the stream and increases in open-water areas.

Groundwater inflow to Johnson Creek was identified by seepage measurements (fig. 4). A similar stream temperature is shown in figure 7 between RM 7.8 and 5.5, where little groundwater enters the stream. In contrast, groundwater discharge between RM 5.5 and 3.2 contributed to a decrease in the temperature of Johnson Creek of about 2–3°C.

Stream temperature can increase in ponded and open-water areas. Although the flow of Crystal Springs Creek originates as cool groundwater discharge, this stream warms because of exposure to relatively large and shallow open-water areas. Summer warming between the site at Bybee Boulevard (map number 39) and the site at the mouth of Crystal Springs Creek (map number 40) is a result of the configuration of the stream channel in Westmoreland Park where the stream flows through a shallow pond. The 7-day moving average of the daily maximum stream temperature in 2005 is shown in figure 23. Warming of Crystal Springs Creek between the Bybee site and the site at the mouth of the creek is about 2°C.

The thermal effect of tributary inflows to Johnson Creek depends largely on the relative volume of flow. Although summer temperature of Kelley Creek (map number 29) is low compared to Johnson Creek (fig. 23), streamflow also is low compared to Johnson Creek. Kelley Creek, entering Johnson Creek between the Gresham (map number 26) and Sycamore (map number 30) sites, probably has little effect on stream temperature at the Sycamore site. In contrast, Crystal Springs Creek contributes most of summer streamflow of Johnson Creek at Milwaukie (map number 41) (fig. 4). Summer temperature of Crystal Springs Creek at the mouth (map number 40) usually exceed that of Johnson Creek at Milwaukie (fig. 23). Much of the 1–2°C increase in the temperature of Johnson Creek between the Sycamore and Milwaukie sites is attributed to inflow from Crystal Springs Creek.

Summer temperature of Johnson Creek is largely controlled by air temperature. Comparison of stream temperature to air temperature during the warm-weather period (from May to October) of each year provides a reliable relation. A regression relation was developed for each year. The 7-day moving average of the average of the daily maximum and minimum air temperature at the Portland Airport (Oregon Climate Service, 2007) was compared to the 7-day average of the maximum stream temperature at the Gresham and Milwaukie sites. This relation was fairly consistent within each year at each site. The period of record at the Gresham site was from 1999 to 2006, and the average r² value for each year was 0.89. The record at the Milwaukie site began in 1998, and the average r² value was 0.90.

Although the relation of air temperature to stream temperature was fairly consistent from year to year at the Gresham site, the relation at the Milwaukie site was more variable. Estimated stream temperatures resulting from an average air temperature of 22°C are shown in figure 24. At the Gresham site, the predicted stream temperature at that air temperature varied by less than 1°C from 1999 to 2006. At the Milwaukie site, a shift from the 1998 to 1999 period to the 2000–2006 period indicated warmer stream temperature for a given air temperature. The shift is attributed to the large decrease in groundwater discharge and subsequent reduction in flow of Crystal Springs Creek. The streamflow of Crystal Springs Creek in 1998 and 1999 was about 17 ft³/s (fig. 2). Streamflow of Crystal Springs Creek ranged from about 10 to 14 ft³/s from 2000 to 2006. This decrease in streamflow resulted in decreased stream velocity, and overall, greater warming in open-water areas of Crystal Springs Creek, resulting in warming of Johnson Creek at the Milwaukie site for a given air temperature. The summer temperature of Johnson Creek at the Milwaukie site at an average air temperature of 22°C increased by about 1.5°C in the 2000–2006 period compared to the 1998–99 period.

The thermal effect of groundwater discharge and open-water areas on stream temperature may provide insight into meeting stream-temperature standards, informing land-use decisions, and focusing restoration efforts. Land-use decisions that foster increased infiltration (and subsequent groundwater discharge), limit instream withdrawals, and increase nearstream shading may improve the summer temperature regime in Johnson Creek. Restoration efforts focused on elimination of instream ponds and increases in tree canopy particularly in areas of highest summer flows may lead to decreased summer stream temperature. Continued monitoring of groundwater levels, of streamflow, of temperature, and tracking the many restoration activities underway in the Johnson Creek basin may provide insight into the effectiveness of measures taken to achieve stream temperature goals.

Sediment and Other Contaminants

The area of the Johnson Creek basin upstream of the Sycamore site contributes more streamflow per-unit-area than the area below this site, where peak flow and volume was large relative to peak streamflow and volume as measured at the Milwaukie site, considering the increase in drainage basin size. Sediment can be a water-quality concern causing problems for fish and other wildlife. Tanner and Lee (2004) showed a relation between organochlorine pesticides and suspended sediment in Johnson Creek. A higher concentration of pesticides per unit of sediment was measured in the area of the basin upstream of the Palmblad Road site (map number 25) compared to the area of the basin upstream of the Sycamore (map number 30) and Milwaukie (map number 41) sites. These results show the importance of managing runoff and minimizing entrainment of sediment in Johnson Creek, particularly in the upper basin.

Land-use activities are part of development in urban, agricultural, and previously undisturbed settings in the Johnson Creek basin. Understanding the transfer of precipitation on the landscape to streamflow, and the sources and fate of sediment and other contaminants in the basin will aid jurisdictions in making land-use decisions for protection of water quality.