This project is an analysis of the changing characteristics of river flows in the Munster Blackwater catchment. The portion of the Blackwater catchment above the river flow gauge at Kilavullen was the focus of this study. This is amongst the oldest continuous flow stations in the country with continuous flow data available from 1955 to the present day. Indeed, it can be said that prior to the 1950’s Ireland did not possess a formal continuous flow gauging network, which inhibits the examination of long term trends in Irish river flows. To examine the long-term characteristics of flow at the Kilavullen gauge, the 54 years of flow data that are available are insufficient and a method of extending the flow record at the site was required. Fortunately, organised precipitation and temperature measurements in Ireland date back further than their river flow counterparts. Within this study digital and historical paper based precipitation and temperature records from Met Éireann were compiled for the area in and around the Kilavullen catchment extending back to 1926. This data was quality controlled and where necessary was used to synthesise historical values of potential evapotranspiration before making it fit for purpose for hydrological modelling. This historical data was then used as inputs to the HYSIM and IHACRES lumped conceptual rainfall-runoff models and used to reconstruct flows (hindcast) at the Kilavullen gauge from 1926 to 2009. This hindcast effectively extended the record at Kilavullen from 54 to 84 years (representing a 55% increase in the period of record). Rather than relying blindly on the rainfall-runoff modelling to reconstruct historic flows, a database of historic floods in the catchment was used to validate the hindcasted flows. Tests for gradual trend in the data show statistically significant persistent positive trends in Annual temperatures, rainfall and flows within the catchment. Evidence of trend in spring flows was found to be the overall driving factor of trends in annual flows. Overall results of the trend analysis on the reconstructed flows general shows that in over the last 84 years, floods in winter months are becoming more common, and that floods in summer months are less evident.