Hi Dave,
With River water, a lot depends on where you live. If you are 'lucky' enough to live in an area say in Dartmoor, Lake district where you KNOW the water is not going to be contaminated with agricultural run off such as fertilizers etc or that the river does not originate or cross limestone, then you will be safe.
Personally, I would not risk it with river water. Your safest cheapest bet in the long run is a water butt, and depending on how many plants you have, I calculate with 3 to 4 litres per plant water storage to outlive a 6 week dry spell which we can and do get in the UK. To give you an idea, I have 10,000 litres water storage as my collection is extremely valuable.