The EISCAT Svalbard radar (ESR) monitors plasma parameters in the ionospheric region that is frequently located near the polar cap boundary. The SuperDARN radar at Hankasalmi, Finland, detects coherent echoes from this region, and these echoes typically show increased spectral width. We consider data of joint ESR and SuperDARN observations to show that the spectral width of HF echoes tends to increase with the ionospheric electric field. This relationship is explained in terms of nonlinear evolution of the E × B gradient drift instability with energy cascade from hundreds of meter wavelengths to meter wavelengths. We assume that decameter waves (seen by the Hankasalmi radar) with relatively large amplitude decay through a three-wave interaction with shorter wavelengths and estimate that the decay time of the decameter waves/irregularities is determined by the parameters of the shorter-wavelength structures. We associate the decameter wave decay time with the correlation time, and thus the spectral width, of HF echoes.