In this paper, the thermal deformations of Brillouin scattering spectra were experimentally detected with a heterodyne system during operation of a high-power fiber amplifier and theoretically described using a numerical rate-equation model. At low signal intensities the Brillouin gain spectrum was determined with a pump-and-probe measurement. A nonplanar ring oscillator (NPRO) with 2 W of output power at 1064 nm and an emission linewidth of 1 kHz in continuous-wave operation was amplified using a 4 m long ytterbium doped photonic crystal fiber (PCF) with a core and cladding diameter of 22 mum and 225 mum, respectively. The active fiber was pumped counter-directionally to the seed radiation with up to 195 W of launched pump power at 976 nm. A slope efficiency of 75\% and a maximum output power of 148 W were achieved in SBS free operation. A deeper understanding of the Brillouin scattering spectral evolution in fiber amplifier systems will help to increase the power level into the kW range.