Abstract: Riverhead is a source area forming a tributary but it is difficult to determine accurately, so it is often judged by a threshold size of the catchment area above the river starting location (minimum support area). How to define this threshold is a key issue in digitization of river network, directly affecting drainage density and watershed division. Traditionally, a given constant threshold value or some empirical thresholds are used, and methods based on meteorological and hydrological conditions for determination of the catchment area threshold are lacking. This paper analyses the existing methods for determination of the threshold and the main characteristics of riverheads, and presents a new criterion that the river starting location is a cross section with mean flow rate of 0.005~0.010 m3/s in dry season. And by this criterion, we suggest a new method for threshold calculation using three hydro-climate factors: annual rainfall, annual evaporation, and minimum stream flow in arid season. Then, a series of thresholds were calculated by using the digital elevation map (DEM) of the Beishuiqu sub-basin of the Niyang River and extracting the river networks with ArcSWAT, and verified via comparison with a reference river network that had been extracted from a larger-scale digitized map. The results show that for this sub-basin, if a minimum flow Qc=0.0085 m3/s is taken as the critical support flow, a good agreement with the real river networks can be achieved and the corresponding threshold riverhead area is 4.91 km2. The suggested method considers meteorological and hydrological conditions and gives reasonable results, and thus it is significant to extraction of digital river networks in complex hydro-climate conditions.