Photo-degradation has recently been shown to be an important pathway for the conversion of DOM to CO2 in freshwaters. While the fundamental controls on DOM photo-degradation include (1) UV light availability and absorption by DOM, (2) DOM lability to photo-degradation, and (3) water residence time, these controls and their interactions have not been investigated across the range of characteristics of freshwaters. To address this gap, we showed how the characteristic features of headwater streams draining permafrost soils in the Arctic (i.e., high concentrations of light-absorbing DOM in shallow, low relief streams) interact with hydrology to control the degradation of DOM by UV light. In addition to measures of light availability and light absorption by DOM, we showed that differences in DOM concentration and character with depth in Imnavait Creek, a beaded, headwater stream in the Arctic, were likely caused by photo-degradation of DOM. These in-situ measurements of DOM concentration and quality provide support that sunlight is an important control on DOM fate and export in streams like Imnavait Creek. From our in-situ observations and experimental rates of photo-degradation, we concluded that over the range of conditions observed at Imnavait Creek, photo-degradation is limited by the availability of UV light (light-limited), and not by the amount of DOM (substrate) present. We expect that our conclusions likely apply to first-order streams and thaw ponds in the Arctic with features similar to Imnavait Creek, and we provide a conceptual model synthesizing the nature and controls on DOM photo-degradation of a river reach (or whole system), expressed as a function of light attenuation and water residence times. Our conceptual diagram can be used to compare and contrast expected controls on DOM photo-degradation across freshwaters.