In the last chapter, we introduced the different sorts of analytical models we can use to detect impacts. There are distinct and important differences between these models, and yet understanding these differences is only the first step. The next step is to be able to apply these designs sensibly and usefully in real situations. In many cases as we will see, the ecology of streams and rivers (and, indeed, many other environments) is not sufficiently well understood to make perfect or even very good decisions about design in every instance. As we shall argue below, however, the critical issue is to understand how to make good design decisions, and, because monitoring designs always involve compromises, to be very clear and explicit about the reasoning upon which such decisions were based. The remaining part of the process, then, is to understand what monitoring designs can tell you definitively and what they cannot – what can be legitimately and logically interpreted from the data versus what will remain unclear.

Below we describe a hierarchy of decisions that will help define the nature of the problems faced in monitoring impacts in flowing waters properly. There are several problems we have to solve to apply good design principles. These problems are the location and character of control locations, and the frequency of sampling through time. In many places, we will suggest that a systematic and well-structured review of the literature will be necessary to solve these problems.