Most streams draining urban landscapes are ecologically degraded. A primary source of this degradation is urban stormwater runoff, which severely alters flow and water quality regimes. It is hypothesised that the hydrological impacts of stormwater can be mitigated by improving the sustainability of drainage practices. This involves catchment-wide application of stormwater source-control measures (SCMs)—e.g. rainwater tanks, rain-gardens, infiltration systems, etc. The Little Stringybark Creek (LSC) Project (see www.urbanstreams.unimelb.edu.au) is one of the few projects in the world to test if SCMs can influence catchment-scale hydrology. Since late 2008, hundreds of SCMs have been constructed in two impact catchments. In parallel we have been monitoring the streamflow in the impact catchments as well as in nearby reference and control streams. The reference streams are free of stormwater impact, whereas the control streams are ecologically degraded and have not been subject to intense SCM implementation. In this work we assessed whether the application of SCMs in the impact catchments has had a detectable hydrological impact at the catchment scale. To do this, we firstly selected thousands of individual storm-events from the rainfall-runoff records. For each storm event, we calculated flow metrics of interest—e.g. runoff coefficient (runoff divided by rainfall). We hypothesised that the sustainable drainage practices have reduced the amount of rain which becomes runoff in the impact catchments. We carried out the assessment using Bayesian hierarchical modelling in order to account for differing degrees of manipulation among impact catchments in relation to reference and control catchments. We present here the preliminary results from this statistical modelling. These results suggest that such interventions have greater impact on runoff behaviour than they do on baseflow behaviour, reflecting the complexity of factors driving low flows in urban catchments. This work has important implications for the way stormwater is managed in our catchments for the projection or restoration of receiving waters.