Estimating the power of detecting a change in the level of contaminants under various sampling strategies in a river poses many challenges and assumptions. We discuss a programme of work to estimate the standard errors for detecting change over time using currently sampled sites. We extend our analysis to predict the errors in unsampled river reaches. Hence an estimate of the power, given a level of change, the sampling frequency, and the time-period of sampling, can be found. Models are required for the data for each river reach over time, for the standard errors using meta data for the river reaches and for the future scenarios. Each of these will be discussed and various options presented. Results for a range of indicators for New Zealand river reaches of stream order 3 and above as examples to assess the usefulness of the tool will be presented.