UC Davis Department of Plant Sciences

RIPARIAN ECOSYSTEM RESPONSE TO TIMBER HARVSESTING FOR THE PURPOSE OF RESTORING ASPEN


Stream Temperature

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Figs. 7 – 9 report the 7-day running average daily maximum water temperatures observed on Pine, Bogard, and Bailey Creeks upstream and downstream of the treatment areas. Q = mean discharge (cfs) from Jun 15 – Aug 31, P = percent of mean annual precipitation.

Stream temperature appears to be driven primarily by discharge, with the highest stream temperature occurring during the lowest flow years.

Minimizing increases in stream temperature is a critical part of maintaining riparian ecosystem health for stream biota. Studies have found that the optimal temperatures for rainbow trout (Oncorhynchus mykiss), which is the native trout species in the region, range from 60.8 to 64.4 o F.

Figs. 7 – 9 show that the maximum stream temperatures were almost always within the optimal range during the course of this study. The highest temperatures observed at any of the sites throughout the course of this study occurred in Bogard Creek during the low-flow years from 2007 through 2009. Despite these relatively high temperatures, a different study found an abundance of trout in Bogard Creek during this time period, reflecting the suitability of these creeks to provide habitat for cold water fish even with conifer harvesting activities.

Figure 7. (Right) Seven day running average daily maximum water temperature (°F) on Pine Creek. Station PC11 was located immediately upstream of the areas treated during Phases 1-3. Stations PC10 and PC8 were located immediately downstream of the areas treated during Phase 1 and during Phases 2-3, respectively.

   

 

       

 

 

These is a general pattern of increased temperature from upstream to downstream stations along each creek (Figs. 7 – 9). Although the rate of increase in stream temperature from upstream to downstream stations varied annually, it was not associated with conifer removal treatments.

For example, there was no change in the rate of increase from PC4 to PC3 or from BO5 to BO3 following Phase 1 conifer removal in January 2004.

Additionally, the rate of increase from PC4 to PC1 showed no response to Phase 2 or 3 treatments. The rate of increase from BO5 to BO1 rose in 2006 and 2007, following Phase 2 conifer removal, but this treatment is unlikely to be the cause of the rise, given that it had no significant impact on either canopy cover or solar radiation. The rate of increase from BO5 to BO1 then declined in 2008 following Phase 3 conifer removal.

At Bailey Creek, the rate of increase was similar both before (2003-2006) and after (2007) treatment (Fig. 9).  It then rose in 2008, but declined again in 2009 and 2010.

Figure 8. (Left) Seven day running average daily maximum water temperature (°F) on Bogard Creek. Station BO6 was located immediately upstream of the areas treated during Phases 1-3. Stations BO4 and BO2 were located immediately downstream of the areas treated during Phase 1 and during Phases 2-3, respectively.

 

 

       

 

A temperature response was expected to the Pine-Bogard Phase 3 and Bailey treatments because they occurred directly adjacent to the creeks and significantly decreased canopy cover and increased solar radiation. There are several possible reasons for the lack of response:

(1) the decrease in canopy cover was not very high at Pine and Bailey Creeks (9 % and 7 % decrease, respectively);
(2) there was still a substantial amount of canopy cover remaining at Pine Creek (55 %), Bogard Creek (39 %), and Bailey Creek (45%) which likely provided sufficient shading to continue to moderate stream temperature;
(3) at Bailey Creek, stream temperature change is likely buffered by the relatively high, cool flows that characterize the creek all season-long;
(4) it is likely that the affected reach lengths at each creek were not long enough to allow for a water residence time that could result in increased temperatures. This may particularly be the case at Bogard Creek, where there was a 35 % decrease in canopy cover, and an increase in temperature was expected.

For the purposes of Aspen restoration, the scale of the conifer removal treatments in this study are among the largest that would be carried out should this method beapplied throughout the region. This implies that the future application of these aspen restoration treatments would be unlikely to increase stream temperatures

Figure 9. (Right) Seven day running average daily maximum water temperature (°F) on Bailey Creek immediately upstream (BR1) and downstream (BR6) of the areas treated during the Bailey Project.

   

 

   

 

 

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