I am doing a geography essment about the river severn, any good sites?

I am in year 8 and we have to make a leaflet about a canoe trip down the river Severn. We have to explain all about the source and how the river severn was formed, as well as what part the canoe starts from. I am probably not making much sense here, the reason being is that I don't understand my homework very much, but I need some sites that might give me some information about the River Severn, how it was formed, and anything else you think a leaflet about a canoe trip should include. Thanks