The River Severn Navigation starts at Pool Quay, near Welshpool, and ends where it enters the sea in the Severn Estury.
This Guide Covers the section covered by the British Waterways licence. There are locks that are manned, and the river is wide enough so that several craft can pass each other at the same time.
There are signs at the locks indicating the river levels. GREEN, Normal navigable level, ORANGE Increased current, proceed with caution and RED Danger - stop and moor your boat (River in Flood)
The river is very rural, with few locks. Making it a ideal place to paddle your canoe/kayak. Dont shoot the weirs, most will kill you. Though Bevere Weir can be in low water conditions with care.
Though the River Severn is a navigation, it still has a flow. Which increases as river levels rise.
More information is available from British Waterways.