Lucy Stebbings: Allen Banks and Staward Gorge is a lovely National Trust site. The river is stone & boulders and mostly of paddling depth but at a bend, overlooked by the steep, tree-lined gorge sides, the river has cut out a deeper pool suitable for a short swim.
The area just upstream is shallow and good for kids to paddle in and there is a stony shore on the inside of the bend that makes a nice picnic site. Park at Allen Bridge National Trust car park (signed from the A69 between Haydon bridge and Bardon Mill - there is a car park fee for non-trust members) and walk approximately three quarters of a mile up stream on the west bank towards Plankey Bridge past the suspension bridge. At the right hand bend in the river, there is a stony beach and its deep enough to take a dip. I swam there on a sunny day mid-september and it was tingly cold and very refreshing, with a light current.
Some submerged rocks so watch your knees and careful about jumping. Judging by the debris lodged in surrounding bushes the river probably gets quite wild after heavy rain.