The summer thermocline of Lake Champlain, which is found at depths of 20--30 m, oscillates with typical vertical amplitudes of 20--40 m and periods of ~4 days. Fluctuations at the ends of the lake are opposite in phase and accompanied in the central lake by strong shears across the thermocline. These are basin-wide baroclinic disturbances which are forced by wind. A numerical, one-dimensional, two-layer, shallow-water model incorporating nonlinear and frictional effects in a rectangular basin forced by wind was first tested with idealized wind impulses. The results do not resemble the observed thermocline motion. However, when this simple model is forced with wind data from a nearby shore site, there is reasonable agreement between the model results and observed long-period thermocline motions in Lake Champlain. Dispersion effects appear to be negligible here. This contrasts with other long, narrow lakes where dispersion effects are important and internal surges are followed by wave trains resembling the soliton solutions of the Korteweg-deVries equation. A possible explanation for the different regime in Lake Champlain may be found in its unique bathymetry with sloping bottom at the ends and numerous embayments on the sides that provide traps to collect wind-driven warm water and then release it slowly during recovery of equilibrium, preventing the formation of steep fronts and soliton wave trains. ¿ 1998 American Geophysical Union