Many urban streams (as well as rural ones) suffer from modifications that place barriers between different pieces of habitat. Very often the habitat that fish use as adults is some distance from the habitat that they use to reproduce. What happens when there is a barrier between the two? And if you think that this is only possible in the USA - have a word with the people at Chester-le-Street Angling Club who completed partnership project work (with the Wild Trout Trust as one partner) to install flow baffles in the base of previously impassable culverts. They now have good numbers of sea trout spawning upstream in places that they could not previously access.