It is found in the eastern Pacific, specifically from Tee Harbor, Juneau in Alaska to the Sacramento–San Joaquin drainage in California, USA.

[3] Adult western river lampreys are noted by their long body, round mouth, and lack of jaws.

This structure helps facilitate the more predatory feeding style of the river lamprey (more below in the Diet section).

[9] The river lamprey is considered one of four species in the Lampetra genus found in the western United States.

[12] Research that examined the pancreas to distinguish lamprey species found differences in their pancreatic tissue, however the authors noted this could be due to genetic variation.

[12] However, more recent genetic research suggests the western brook lamprey would be better categorized as a member of a Lampetra ayresii species complex.

[3] During this time river lamprey primarily feed on muscle tissue of American shad, Chinook salmon, Shiner perch, and Pacific herring.

[7][14][15] The large wound size present on prey can be explained by the river lampreys feeding style.

Starting their lives as an ammocoete (larva), they burrow into the stream bottom and survive as filter feeders.

They spend a parasitic feeding phase here for 10 weeks, during which they grow rapidly and reach their maximum size.

[9] During their freshwater stages, river lamprey can be found in the main tributaries between the San Francisco Bay and Juneau, Alaska.

While many of these dams do have screening methods to capture and release migrating fish, many of these systems are designed primarily for salmonoids.