HOW IMPORTANT ARE CHANNEL HEADS? ANALYSIS OF HILLSLOPE-CHANNEL CONNECTIVITY IN MORPHOLOGICALLY DISTINCT CATCHMENTS

A fundamental component of sediment connectivity within catchments is the pathway(s) through which soil is transported from hillslopes to channels. Defining these transitions and their controls across landscapes with varying topography is critical to understanding landscape evolution. Channel heads are the most well studied form of this transition, but hillslopes communicate with channels through multiple processes and pathways, and little attention has been paid to quantifying their respective role in sediment connectivity. We explore hillslope-channel connectivity in two morphologically distinct catchments located within the Sapphire and Bitterroot Mountains in western Montana. We investigate controls on channel head zone morphology by mapping channel head features and analyzing local measurements of topographic variables such as slope, curvature, and surface roughness. We then quantify the degree of connectivity of zero-order catchments relative to other hillslope compartments by mapping hillslope zones, producing topography-based index of connectivity maps, and coupling these data with soil depth and cover measurements. We find that zero-order drainages consistently comprise less area compared to other hillslope zones in both of our systems, though topographic differences affect the magnitude of this relationship. Additionally, channel heads can be defined by a discontinuous zone where features begin and end several times before establishing a continuous channel, thus representing multiple interruptions of soil and sediment transport. This differs from the common view of a single, discrete channel head. We find that these discontinuities are more common in the Sapphire location, where surrounding slopes are gradual compared to the steep terrain in the Bitterroot catchment. Our results provide quantitative insights to where hillslopes and channels communicate and how this is affected by topographic variations within a catchment as well as between landscapes. The transfer of sediment between geomorphic domains has implications spanning multiple disciplines by influencing ecosystem functions and impacting land and resource management.