Kargopol is located just north of where the River Onega flows out of Lake Lacha.  From the Medvezhy Krai Eco-Park it is around 4km to get to a small beach on the lake. However as the eco-park itself is rather far from the city centre, it is probably best to rent bikes if you are planning on going there.  It is worth the effort of getting there to enjoy the practically completely unspoiled views of the lake.  Be warned though that in summer the horseflies here are merciless and insatiable.