Observations of isotherm displacements in a lake through which a large river flows have been used to study the behavior of short-period internal waves. Selected time series as well as spectra suggest a generating mechanism for short-period internal waves localized in the region of the inflowing water which may be associated with the formation of a mixed zone imbedded in the stratified column. The mixed zone, in turn, is considered to originate from turbulent instability due to vertical shear associated with the river inflow and long-period internal seiches. The model for the collapse of a mixed region by Schooley and Hughes (1972) accounts for observed spectral peaks at frequencies lower than the Brunt-V¿is¿l¿ frequency.