Kootenay Lake is a long (100 km), deep (140 m), dimictic lake in southeastern British Columbia. Internal waves were observed over an 18-month period using three thermistor chains moored along the major axis of the lake. To characterize their seasonal aspects, four types of oscillations are recognized: (1) classical seiches with amplitudes of 10-30 m and periods of 1-2 weeks, (2) intrabasin waves with periods of 2-4 days, (3) large-amplitude waves resembling nonlinear surges, and (4) high-frequency waves associated with both the internal surge and large-amplitude internal seiche motions. The periods of the major components of the internal wave field are determined mainly by morphometry but vary seasonally with stratification. Thus the wind, which not only forces internal waves but can also modify stratification, is important. In large lakes like Kootenay, the thermal structure may vary not only spatially but also temporally at periods comparable to those of the internal waves themselves. Hence the shape and frequency of these waves may be altered as they pass through the lake.