As a geologist; atmospheric, surface and ground processes are facts that I have as the base to my knowledge of the overall processes involved in how the “world works” and I find some of the questions that my friends and for that matter my wife’s friends ask to be astonishing in their apparent ignorance. It always amazes me that so many people have such a limited understanding of such things as how a river works.
Anyway, the water flowing down the river seven that you are seeing now will be from a number of sources, the prime source being the water table / ground water. This supplies what is know as the rivers base level, though this does change as the water table changes over the year. The water in the rivers from rain, melt water, sheep wee doesn’t stay in the river for very long the length of time is dependant on the ground cover, geography and geology of the rivers catchment area. Water from a rain storm can be seen in the river in just a couple hours and can increase the water level for half a day after the rain has stopped. The SEPA website has some nice real-time information where the river level can be gauged against rainfall and other factors, its great for picking which rivers have water for kayaking, and the speed that the water level increases following storm rain is interesting to see – look at any rivers data following a prolonged dry spell and you will still see water flowing, which is sourced from its ground water supply, which has already been explained wont be frozen, as in our climate even during really long cold spells the level to which the ground freezes only extends to a few centimetres.