AbstractWhen barriers to migration are removed, anadromous fishes from other rivers may colonize newly accessible habitat or landlocked forms of the species may resume anadromy if conditions allow. For example, two large hydroelectric dams on the Elwha River, Washington, were removed between 2011 and 2014 to restore populations of Pacific salmon Oncorhynchus spp. after a century of isolation. Immediately prior to and following dam removal (2010–2017), upstream‐migrating adult Sockeye Salmon Oncorhynchus nerka were sampled, and DNA was extracted and compared with samples from the local Lake Sutherland population (above the lower dam’s former location) and a coastwide genetic baseline to determine the probable origin of each fish. Genetic analysis indicated that none of the 45 Sockeye Salmon sampled had originated from the local population or other populations nearby in the genetic baseline, and only a single fish originated from the abundant and proximate Fraser River populations. The strays reflected two different sources and processes: 22 were from the west coast of Vancouver Island (all in 2015, when these populations were very abundant and encountered conditions unfavorable for upstream migration in their natal system) and another 22 assigned to more northerly populations, mostly to riverine populations from the Stikine River. Test fishery samples collected in the Strait of Juan de Fuca contained very few fish with assignments similar to these northern rivers. It is not clear whether these individuals originated from the distant riverine populations or from a closer population not in the baseline that resembles them genetically. Taken together, the findings indicate that the genetic composition of any future population in the Elwha River may reflect signatures from diverse sources and subsequent selection in the new habitat.