The Tweed in Scotland is a very clean river and is home to a range of wildlife, including salmon. This draws anglers to the river. The area around the Tweed is home to the Scottish textile industry. Mills were set up along the river, with water providing the power needed to run them.
The class could be posed the question: "What are rivers for?" They could even be told (like in the clip) that a visiting alien has come to Earth and this is one of the questions he has about our landscape. The clip could then be used as an example of an answer before children are set off to write their own answers for the alien.