Do tarps depend on trees? Not everywhere has them... Where I live, once the river takes you out of the city, it's bordered by agricultural land. Fences or hedges, crops or grass + animals. Very occasional groups of trees near the water, mostly on land unsuitable for farming, e.g. steeply sloping - you might get a tarp up, but you'd need to combine it with a hammock for sleeping. And such uncultivated patches tend to be full of stinging nettles and/or wild blackberry shrubs, nature's barbed wire. "Wild" camping is not legal in England, but if I were going to try it, I'd go for a green tent plus a groundsheet, and try to find a level spot by a hedge.
The legal way is to use organised camp sites. There are lots, but not always conveniently by a river - most of their customers arrive by road.
Wild camping is legal in Scotland, with a few sensible restrictions to stop you pitching in somebody's garden or damaging crops.
Not in Oxford any more...