Background/Question/Methods   Increasingly, ecologists need to identify and quantify relationships between landscape gradients and aquatic ecosystems. Considerable statistical challenges emerge in this effort, some of which are attributable to covariation between human development and landscape gradients. Both Johnson and Gage (1997) and Allan (2004) identify difficulties caused by the covariation between anthropogenic activities and natural landscape gradients. In this paper, we measure the covariation between human development---such as agriculture and urbanization---and natural landscape gradients---such as valley form, climate and geology. We then show how this inherent covariation undermines statistical efforts to measure mechanistic links between landscape gradients and significant features of aquatic ecosystem, such as fish abundance.

Results/Conclusions   With linear regression, we quantify covariation between human activities and landscape gradients within a dataset of stream sites from the Oregon Coast, USA. We show that the correlation between human development and natural landscape gradients varies dramatically with the scale of observation. Similarly, we show how the correlation varies by sub-region, even within a scale of interest. We then illustrate the negative consequences of this underlying correlation structure for statistical efforts to measure indicators of healthy river systems, e.g. juvenile coho salmon distribution and abundance. We conclude by discussing how to address this challenge in the design of monitoring programs and statistical models.