The Allen was formerly very polluted from mining activity upstream, but remediation schemes have been implemented to allow the water quality to improve.

[3] The river is classed as the Allen from a place called Water Meetings, which is just south of Cupola Bridge, a grade II listed structure built in 1778.

However, this in turn meant that more wildlife was noted around the river, including red squirrels, roe deer and more otters.

[17] However, further downstream on the River Tyne, the metalliferous pollution has promoted certain types of grasses to grow which thrive on lead and zinc ores in the water.

Due to the amount of water that can be drained into the river, the steep nature of the valley walls and the "impervious geology" of the underlying rock, the river is noted to be a problem for flash flooding, which can lead to inundation problems downstream.