State scientists, grappling with an explosion of marijuana growing on the North Coast , recently studied aerial imagery of a small tributary of the Eel River , spawning grounds for endangered coho salmon and other threatened fish. In the remote, 37-square-mile patch of forest, they counted 281 outdoor pot farms and 286 greenhouses, containing an estimated 20,000 plants — mostly fed by water diverted from creeks or a fork of the Eel. The scientists determined the farms were siphoning roughly 18 million gallons from the watershed every year, largely at the time when the salmon most need it.