Although 'officially' beginning at the A34 road bridge, the lower reach of the River Kennet is perhaps better defined for paddling purposes from Newbury.
Following a 1715 Act of Parliament, The Kennet Navigation was created in the period from 1718 to 1723. The task of making the River Kennet navigable between High Bridge in Reading through to Newbury involved digging an additional 17.7km (11 miles) of locks and cuts which alternate with the natural river bed, thus avoiding some of the more tortuous detours and bypassing sections where mills straddled the waterway making it impassible to cargo carrying vessels. Between Tyle Mill Lock near Sulhamstead and County Lock in Reading this work permitted navigation over a drop of nearly 40m.
Today, the Kennet Navigation is administered by British Waterways, who regard it as part of the Kennet and Avon Canal. A licence is required for paddling. From 1 April 2009 to 31 March 2010 the fees are shown here, and have increased with the single option price of a 12 month licence for £39.52.
Whether 'the navigation' includes the non-canalised sections of the river between Newbury and Reading is an issue that will probably best be determined by your views on the rights of navigation in general. When the question was put to British Waterways the following written answer was received:
"We are responsible for the canalised sections of the River Kennet where they form part of the Kennet & Avon Navigation only. We believe that the historic riverine sections are the responsibility of the Environment Agency."
In late December 2009 the Department for Environment, Food and Rural Affairs (Defra) published its consultation on the Government's strategy for the Inland Waterways of England and Wales, titled "Waterways for Everyone". This document clearly states (in section 2.7 on page 9) that with regard to the Environment Agency "All its waterways are rivers with a public right of navigation".
Below the start of the 'new' navigation at High Bridge (65m (70 yards) downstream to be precise) down to Kennet Mouth where the River Thames is joined, this section of the River Kennet (which has been in use as a navigation for over 700 years) is administered by the Environment Agency as part of the Thames. Details of registration of unpowered vessels i.e. canoes and kayaks, can be found here.
Membership of the British Canoe Union - click here for details - includes the cost of both licences. Basic adult membership is £32.75 per year (as of March 2009).
A paddler moored at Bonsal Point.
The River Kennet will occasionally flood on the lower reaches with the last event being recorded after the snow melt in 2009 - see here and the later images of county Weir in the center of Reading.