Timber harvest and topographic effects on the intermittency of headwater streams in western washington

Abstract

Headwater streams make up the majority of the total stream length in the state of Washington. These non-fish-bearing upper reaches are critical habitat for sensitive stream-associated amphibian populations. During the low-flow summer months these streams contract and the surface flow that amphibian species rely on becomes disconnected from other flowing reaches, sometimes by hundreds of meters of dry channel. To understand these spatial and temporal shifts in the dry reaches of the headwaters, I used data that we collected as part of the Soft Rock study for the Washington State Department of Ecology. We measured the length of every dry reach in 11 non-fish-bearing watersheds, located in southwest Washington, for 8 years (2013-2020). In 2014 and 2015, a clearcut timber harvest treatment was implemented at 8 of the 11 basins. We used a generalized linear mixed model to test for a treatment effect on the percentage of the stream network that retained surface flow. In the first 2 years post-harvest, we found a 26% (post-1) then a 24% (post-2) increase in the Percent Wetted Channel (PWC) relative to the reference sites. In addition to treatment effects, we also tested for precipitation and topographic impacts on the PWC using a Pearson's R and a Random Forest model, respectively. Summer precipitation was correlated with PWC at the reference sites whereas annual precipitation was not. We also found synergistic effects with some of the topographic features. Smaller drainage area, low-gradient valleys, and steeper stream slopes were all associated with drier sections of the streams. All of these features are more likely to be found higher up in the watershed, so it is probable that a combination of this topography is increasing intermittency in the upper reaches of the headwaters of western Washington. This contraction and disconnection of surface flow at inconsistent spatial and temporal scales could create unreliable habitat for stream associated amphibians. Further research is needed on the habitat implications, and the potential for stream temperature increases due to the greater amount of surface flow post-harvest.