The interconnections between groundwater and river systems remain poorly understood in many catchments throughout the world, and yet they are fundamental to effectively managing water resources. Groundwater extraction from aquifers that are connected to river systems will reduce river flows, and this has implications for riverine ecosystem health, water security, aesthetic and cultural values, as well as water allocation and water management policies more generally. The decline in river flows as a consequence of groundwater extractions has the potential to threaten river basin industries and communities reliant on water resources. ¶ In this thesis the connectivity between groundwater and river systems and the impact that groundwater extractions have on river flows were studied in one of Australia’s most developed irrigation areas, the Namoi River catchment in New South Wales. ¶ Gauged river reaches in the Namoi River catchment were characterised according to three levels of information: 1) presence of hydraulic connection between aquifer-river systems; 2) dominant direction of aquifer-river flux; and 3) the potential for groundwater extraction to impact on river flows. The methods used to characterise the river reaches included the following analyses: 1) a comparison of groundwater and river channel base elevations using a GIS/Database; 2) stream hydrographs and the application of a baseflow separation filter; 3) flow duration curves and the percentage of time a river flows; 4) vertical aquifer connectivity from nested piezometer sites; and 5) paired stream and groundwater hydrographs.