Instruments were developed and were used to obtain measurements of the most important flow parameters within self-aerated flows on the spillway of Aviemore dam. Compressibility effects within self-aerated flows are examined. It is shown that Mach numbers as large as 2 will occur in localised aerated regions on large structures. A probe was developed which measured stagnation pressure and air concentration. The compressible nature of self-aerated flows is taken into consideration in formulating a relationship between these measurements and the velocity of the water. This expression is found to overestimate the velocity by about 7% over a wide range of flow conditions. A velocity probe was also developed, based on a cross-correlation technique. This latter probe was the more successful and is recommended for future measurements within self-aerated flows. Profiles of stagnation pressure, air concentration and velocity were measured at each of five positions down the spillway for each of two discharges. These are the most comprehensive and detailed measurements yet obtained on a large structure. The regions of non-aerated, partially aerated and fully aerated flow down a spillway are distinguished. A dimensional analysis is used to indicate the important variables effecting the distribution of air downstream of the point of inception. This forms the basis for plotting the measurements from Aviemore plus the available measurements from laboratory flumes. The shear stress on the spillway surface at Aviemore is calculated from the measurements.