The Harlequin Duck (Histrionicus histrionicus), a striking sea duck well adapted to life in turbulent waters, is a species of conservation concern in much of its range and is designated as a Species of Greatest Conservation Need in Oregon. As North America’s only species of waterfowl that strictly breeds on rapid and remote mountain streams, its vulnerability to climate change, habitat degradation or disturbance, and human activities is heightened by multiple unique life history traits. Despite rigorous surveys in the 1990s and early 2010s, recent monitoring efforts in the Oregon Cascades have diminished, leaving significant gaps in population and distribution trends. Our research aims to address these gaps by evaluating detection probabilities for traditional ground-based foot survey methods and camera traps, while investigating additional non-invasive technologies in environmental DNA sampling and passive acoustic monitoring. We also investigated Harlequin Duck associations with benthic macroinvertebrate densities to evaluate one aspect of habitat use at two historically known breeding sites in Linn and Lane Counties, Oregon. Detection probabilities of visual surveys (0.25; p=0.812) and camera traps (0.28; p=0.010) were comparable, eDNA detections show promise, but acoustic monitoring yielded no confirmed detections. Macroinvertebrate density showed a non-significant, negative trend with Harlequin Duck presence that warrants further investigation. This work provides practical insights into sampling designs for Harlequin Duck studies that can improve conservation planning by wildlife managers.