It is argued theoretically that internal oscillations in a fjord lake have a three-dimensional nature during the stratified period. A dynamical model is proposed that is based on the approximation that for Kelvin waves the component of flow parallel to the isobaths is much larger than the normal component. A general method of solution of the model equations is outlined which takes into account the details of the shoreline configuration, bathymetry, and density structure and is applied to the long-period oscillations of the thermal structure of Kamloops Lake. Siz episodes of internal seiches of the first vertical and longitudinal mode were examined during the period from May to November. Numerical computations predict that periods range from 33 to 140 hours that the largest isotherm displacements occur in the region of slower phase speed in the western portion of the lake. These findings are confirmed both by visual inspection and spectra analysis of the isotherm displacements at three locations in the lake and by wind records. The unusual characteristics of the isothermal displacements at three locations in the lake and by wind records. The unusual characteristics of the thermal structure in the lake are shown to exert a stabilizing influence on large-amplitude wave disturbances. Long-period internal waves are generated primarily by suddenly imposed and vigorous wind forcing along the axis of the lake.