Arctic landscapes are exhibiting widespread changes as a result of rapid warming and intensifying disturbance regimes in the permafrost zone. Large-scale permafrost thaw, soil active layer expansion, and deepening subsurface flow paths are expected to facilitate the lateral transport of old dissolved organic carbon (DOC) to Arctic rivers, though additional research is needed to characterize variability in sources and composition of riverine DOC that impact its lability and turnover time. For example, DOC leached from newly thawed permafrost soils can provide a highly labile carbon source for microbial processing to CO2 and CH4. Therefore, estimating variability in the sources and structure of carbon entering Arctic surface waters remains a key uncertainty in understanding changes in the net ecosystem carbon balance of the permafrost zone. In this study, we repeatedly sampled ~40 nested subcatchments within three ecologically distinct catchments in the North Slope of Alaska throughout the 2021-2023 thaw seasons to characterize spatial and temporal variability in DOC source dynamics within and between catchments. Each sample was analyzed for DOC concentration and optical properties (i.e. composition and lability), and a subset of these samples were analyzed for radiocarbon age. From these data, we implemented two central metrics to understand 1) the influence of individual subcatchments on DOC concentration at the catchment outlet (subcatchment leverage) and 2) the spatial scales that regulate DOC concentration and character variability throughout each catchment (scale of spatial variance collapse). Preliminary results revealed that landscape heterogeneity and seasonal shifts in terrestrial source pools are expressed in riverine DOC concentration and character, and that catchments exhibit unique functional controls on DOC structure and source activation at small spatial scales.