Canaan Valley (hereafter, the Valley), a unique wetland complex set in the Allegheny Highlands of northeastern West Virginia, has been the subject of several investigations by the US Geological Survey (USGS). These projects include studying the surface-water hydrology and processes affecting dissolved oxygen, groundwater hydrology, wetland biogeochemistry, and the formation of peat. Additionally, recent revisions of the region's geologic maps have enhanced our understanding of the Valley's surface rocks. The Valley's streams typically conduct dilute calcium- and magnesium-bicarbonate type waters that are low in alkalinity, nutrients, and dissolved solids. The Blackwater River and its major tributaries, the Little Blackwater River and the North Branch of the Blackwater River, are low-gradient streams. Other tributaries are high-gradient streams that originate on the Valley's sides and fall rapidly to the Valley floor before joining the Blackwater River and the major tributaries. Generally, low-gradient streams are less turbulent than high-gradient streams and dissolved oxygen concentrations are strongly affected by turbulence, re-aeration, benthic photosynthesis, and high biochemical oxygen demand in the numerous beaver ponds through which streams flow and which are present in the Valley. Groundwater in the Valley flows primarily along joints, faults, and bedding planes, and its quality is affected primarily by the mineral composition of the source rock. Septic discharges and, to a lesser extent, land applications of fertilizers and pesticides have affected groundwater locally. The most prevalent contaminants of concern in groundwater are bacteria, radon, and manganese. Nearly half of the wells sampled contained detectable concentrations of fecal streptococcus bacteria, and 25% had detectable concentrations of fecal coliform bacteria. Radon, a carcinogenic gas, was detected in 8 of 12 samples at concentrations exceeding proposed drinking water standards. During periods of stream base-flow, groundwater discharge dominates the flow and influences the chemical characteristics of the Valley's streams. Substrate chemistry, communities of denitrifying bacteria, and plant-community structure were compared among four different wetland types in the Valley. Further wetland studies have estimated the peat resources available in the northern end of the Valley. In this paper, we summarize hydrologic and geologic investigations conducted by the US Geological Survey and others in the Valley over the last 8 decades.