I have been told that one can kayak the entire distance of the river (except the dams) in the spring or after a period of heavy rains.  If you do this set yourself with someone to pick you up at the end. This is not quiet water.   Also it is 37 miles long so it may take more than a day to ride down the entire river.  My experience is that many areas along the river isolated from the casual kayaker.  A few places one actually needs to rent a camping area so a kayaker can put in their kayak in that location.  It is a river of great beauty and soul.

Here is a YOUTUBE video describing the beauty of the river.  It is the first of a series of the purity of the virgin qualities of the river 

Here is a comprehensive guide to the Lamprey River.  Part of the guide describes what to expect boating down the river at different times of the year   http://www.lampreyriver.org/home

This next sight gives current conditions of the river by the USGS.  Don’t ask me how to read it.  http://waterdata.usgs.gov/usa/nwis/uv?site_no=01073500