Chalk streams are watercourses originating from chalk hills, where rainwater soaks into the ground and is trapped by porous chalk rock. The chalk acts as an aquifer, filtering the water that passes through it and creating a clear, mineral-rich spring lower down on the slope. Because the chalk also regulates the flow of water to the waterway below, there is little day-to-day variation in the amount of water entering the stream. Such conditions make chalk streams an ideal venue for what activity? Discuss