Multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors (BSRs) on the northern Cascadia subduction zone margin off Vancouver Island. The reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate. The shallow water depth of 1300 m and the 3600-m-long hydrophone array have allowed BSR amplitude-versus-offset and high-resolution velocity analysis, as well as modeling of vertical incidence data. The results of all three types of analysis can be best explained by a 10 to 30-m-thick high-velocity layer located immediately above the BSR about 300 m below the seafloor, having a sharp base and transitional top. In the layer, about one third of the sediment pore spaces must be filled with hydrate ''ice.'' There is no seismically detectable free gas beneath the BSRs. These results put important constraints on models for the distribution and formation of BSR hydrate. ¿ American Geophysical Union 1992