Somebody will probably come along with better data than I, but there is a dam in Lakeport that controls outflow of the lake.
The lake level is usually maintained within a narrow(ish) band, you can see current and historic data here:
http://des.nh.gov/organization/divis.../winni_levels/
The goal (more or less) is to keep it right around "full lake" the majority of the time. Variables include total rainfall in the area during the summer, snow runoff in spring, and that the dam is adjusted to maintain a minimum amount of waterflow downstream (not sure if there is a maximum).
So, ultimately the level is under human control, for the most part, and the level will try to be maintained as shown on the historic data. You can't extrapolate much from a current level to what it is likely to be at a future point.