The Mann-Kendall statistic is used to investigate trends in homogeneous rainfall and river flow records since the 1860s for 15 basins in the UK. The relationship between the strength of trend and detection time is then explored for seasonal and annual flows. Here it is shown that, under widely assumed climate change scenarios, expected trends in UK summer river flows will seldom be detectable within typical planning horizons (the 2020s). Even where climate driven changes may already be underway, losses in deployable resources will have to be factored into long-term water plans long before they are statistically detectable. Rather than an excuse for inaction, such insights should inform more sophisticated approaches to environmental monitoring, climate change detection and adaptation.