Lupo -
What you found was probably the larval from (called an ammocoete). The adult lamprey spawn in the spring much like steelhead. The construct redds (moving the gravel by sucking on the rock and dragging it out of the way) that look much like steelhead redds. The adults die after spawning like salmon.

We have 3 species here in the PNW - the brook, the river and the Pacific. The larval upon hatching move into the slow water areas and bury in the muck were the feed on the detritus, diatoms, algae and other small items. The ammocoetes spend several years in this stage before metamorphosing into the adult stage. It is likely that this is the stage of critter that you found and were probably washed out their muck home.

The adult brook lamprey is the smallest, typically about 6 inches or so. They spend their entire life in the small creeks and do not feed as adults -spawning and dying short after they become adults. Both the river and Pacific lamprey are anadromous fish which migrate to the ocean upon metamorphosing into an adult. In both cases the adults are parasitic and attach themselves to fish and feed on their fluids. The river lamprey adults are typically 12 inches or so long and the Pacific reach 18 inches or so. Historically they were quite numerous and migrated great distances up the rivers (include the upper Columbia). The numbers of river and Pacific lampreys have declined dramatically over the last couple of decades and may become a cnadidate for an ESA listing. While they can't jump they climb over falls and dams by attaching to the obstacle and pulling themselves up.

Probably more than you want to know!

Tight lines
Smalma