Groundwater is critical in sustaining streamflow, especially in; headwater catchments, because of its ability to supply baseflow. In; water-limited arid and semi-arid mountain environments, the need to; characterize groundwater recharge and discharge has grown in tandem with; demands to manage current and future water resources. However, studying; groundwater in complex terrain is challenging due to limited field; measurements. Nearly a decade of monitoring in Gordon Gulch in the; Colorado Front Range provides an opportunity to study such an; environment. The field data is used to parameterize and calibrate a; groundwater flow model (MODFLOW-NWT). Model results reveal that; groundwater is recharged primarily during one to two recharge periods; each year, driven by spring snowmelt coupled with rain or by; intense/prolonged summer rain. Gordon Gulch is a net gaining stream,; with greater fluxes from groundwater to stream in lower Gordon Gulch and; during springtime. Groundwater is discharged to the stream via long,; deep flowpaths sourced from upper Gordon Gulch and from hillslopes, and; via short, shallow flowpaths in lower Gordon Gulch. Modelled groundwater; accounts for approximately 16 to 34% of baseflow in the stream. Using; Gordon Gulch as a case study, this model and data analysis contribute to; a larger effort to understand and constrain the mechanisms driving; groundwater recharge and groundwater-stream exchanges in semi-arid,; headwater catchments.