EVOLUTION OF A FISHERY CONDITION ASSESSMENT PART III: A MATTER OF SCALE

In the last stage in the Evolution of a Fishery Condition Assessment series, I discussed the importance of individual indicators for describing watershed and fishery condition as well as the qualities that make an indicator ecologically meaningful. Another important consideration in my project work is scale. For our assessment, we therefore have to consider at which scale or set of scales each indicator operates.

Some indicators affect a watershed at the landscape level, while other indicators are only important in the immediate area of the stream. Some indicators may also have greater or less importance depending on the scale that you are considering. In my previous post, I used human-caused and natural effects on sediment regime to illustrate condition. Here I’ll continue to use that example to illustrate why a given indicator might need to be evaluated at different scales.

 Figure 1. Near-stream road sediment loading along Spanish Creek, southeast of Quincy.

Figure 1. Near-stream road sediment loading along Spanish Creek, southeast of Quincy.

Soil type, which is dependent on parent geology, can be mostly similar across vast areas. The quality of soils, including how easily they erode, should therefore be considered at the landscape level i.e. the watershed scale.

Roads, similarly, are distributed across the landscape; there are roads developed everywhere, in a network, that get us where we need to be! So like soils, we should definitely examine their influence at the watershed scale. But, the number of roads has a greater impact in terms of the increasing fine sediment-load in a stream when they are located near the stream itself (Figure 1). So it is necessary to look at roads at another scale that captures this heightened influence on water and habitat quality.

One way to capture the effect of an indicator that is more important when it is closer to the stream is to examine its effect within a buffer. That is, within a given distance to the stream. Using our example, the number of roads within the buffer zone has more weight with regard to water and habitat quality. This is because the fine sediment eroded from the road surface or cut will immediately reach the stream channel whereas at the watershed scale the sediment is captured by vegetation. In order to develop and informed restoration strategy it’s necessary to examine indicators at multiple scales.

In practice, it is often hard to demonstrate precisely which landscape-scale characteristics affect in-stream conditions, yet we know that the landscape and land use across a watershed does affect in-stream condition. The natural landscape and human land use, over time, can act as a constraint or a destabilizing factor for restoration success. Looking at near-stream conditions allows resource managers to act on improving habitat in the short term by implementing restoration. However, doing so in a place where restoration results in lasting success depends on the landscape-scale characteristics that have to be examined at a larger scale.

  Figure 2.   Lower Indian Creek near Greenville, CA displayed at the (A) Watershed (B) Subwatershed and (C) Stream Buffer scales.

Figure 2. Lower Indian Creek near Greenville, CA displayed at the (A) Watershed (B) Subwatershed and (C) Stream Buffer scales.

For the purposes of the UFRBWA, we anticipate looking at the indicators we select at three probable scales (Figure 2). The watersheds of the United States have been divided by the U.S. Geological Survey into a hierarchy based on surface hydrologic features. From this system, we anticipate examining indicators with landscape-level coverage and effect at the “Subwatershed” scale. Indicators such as those associated with projected climate change will likely be examined at the “Watershed” scale, the next step up. For indicators that have the most effect in the near-stream area we will likely developing a “reach scale” that segments streams and evaluates indicators in a buffer zone around those segments.

In short, selecting the right indicators is important for identify overall watershed condition, but because the effects and interactions between individual indicators varies across space and over time, it’s just as much a matter of scale!