Assign each group one of the following rivers on the top 13 list: Missouri River, Canning River, Eel River, Hudson River, Powder River, Mississippi River, Big Sandy River, Snoqualmie River, Animas River, East Fork Lewis, Paine Run, Hackensack River and Catawba River.

CNN: Rivers feel heat as U.S. energy demand rises

The spawning habits of the eel is the exact opposite of the other migratory river users - the salmon and the sea-trout which migrate upstream to spawn.

BBC: River ladders give eels a leg up