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In a laboratory study, we quantified the temporal energy flux associated with the degeneration of basin‐scale internal waves in closed basins. The system is two‐layer stratified and subjected to a single forcing event creating available potential energy at time zero. A downscale energy transfer was observed from the wind‐forced basin‐scale motions to the turbulent motions, where energy was lost due to high‐frequency internal wave breaking along sloping topography. Under moderate forcing conditions, steepening of nonlinear basin‐scale wave components was found to produce a high‐frequency solitary wave packet that contained as much as 20% of the available potential energy introduced by the initial condition. The characteristic lengthscale of a particular solitary wave was less than the characteristic slope length, leading to wave breaking along the sloping boundary. The ratio of the steepening timescale …