Creosote by itself and in large quantities will adversely affect fish, leaching of creosote over time potentially could also--depends on quantity. There a few types of creosote and each one has different properties and levels a toxicity to animals, fish, and humans. Coal tar creosote is harsher than the others. On most power poles and telephone poles you can determine type of tree, creosote concentration/type, and method used.
I would do the jar test just as Rex mentioned and take a water sample to test the properties of the water.
