Temperature Data in the River

Starting in the late summer of 1997, the Water Bureau began monitoring the water temperature in the Lower Bull Run River using thermopod instruments stationed at various locations along the river, Figure 1. The instruments are used for monitoring the water temperature during the summer since this is the time period of most concern for providing cooler water temperatures for salmonid fish. The instruments were deployed in the summers of 1997 - 1999.

Analysis

The thermopod instruments were placed in the Lower Bull Run River in 1997 in only a few locations for the month of September only. Although this provided a limited data set for the summer of 1997 it nonetheless revealed that there was an increase in stream temperature moving downstream from the Plunge Pool to the County Bridge in the early part of September, Figure 2. In the latter part of September the temperature increase seems to have disappeared due to a rainfall event and remained cooler after the event, Figure 3. Figure 12 also reveals that there is a strong diurnal pattern in the water temperatures in the lower river indicating solar radiation has influence on the stream temperatures.
 
 

In examining the data from the summer of 1998, Figure 5, a similar relationship is revealed in the Lower Bull Run River. The water temperature in the river tends to increase moving downstream from the Plunge Pool to the USGS Station during July and continues to have higher peak temperatures moving downstream to Bowman's Bridge. During August and September water temperatures increase from the Plunge Pool to the USGS station but tend to decrease and show less variability between Larson's Bridge and the County Bridge. When meteorological conditions in Figure 4 are compared with the stream temperature data there appears to be a close relationship with rainfall events and cooler water temperatures in the Lower Bull Run River.

Water temperatures recorded in several tributaries entering the Lower Run River also illustrate cooler temperatures in streams higher up in the watershed than streams entering the river lower in the watershed, Figure 6. This would tend to indicate that tributaries entering the river in the lower watershed may result in warmer water temperatures in the river, depending on their flow contribution.

The diurnal pattern seen in 1997 and 1998 is also evident in 1999, Figure 7 and Figure 8. This variation ranges from 1 C^o or less at Headworks and the Plunge Pool to 2-6 C^o at downstream stations. The water temperatures in 1999 increase from Headworks to Larson’s Bridge, then appear to remain steady after that point. Based on a comparison between Stations 146 and 148 (Figure 7) the temperature of the water from the Little Sandy at the confluence with the Lower Bull Run River is 2-4 C^o cooler than the temperature in the Lower Bull Run just upstream. There are periods when the water temperature drops over a period of 1-3 days. For the same time period, these drops are seen at all stations on the Lower Bull Run River and its tributaries.

Data was also collected at Station 145 (Table 1) in 1999. However, because the data is flawed, it is not included in this report.