lake bomoseen is a freshwater lake in the western part of the u.s. state of vermont in the towns of castleton and hubbardton in rutland county. it is the largest lake that lies entirely within the state's boundaries, with a surface area of approximately . the lake was formed by glaciation and has an average and maximum depth of and , respectively. it drains a watershed, has five major inlets, and empties to the castleton river, a tributary of the poultney river, which in turn flows west to east bay at the southern end of lake champlain.