We have modelled, for the cases of Milky Way and M31, the effects on
the galactic discs, of the arrival at high velocity (>150 km/s) of
giant HI clouds, with masses of up to 10^8Msun. Predictions are
compared with the
detailed structure of the observed rotation curves for these two
galaxies. The model explains the rises and falls observed at
large distances from the centre of each galaxy, distributed with
a degree of regularity in radius, in terms of a specific type of
perturbations driven by the infall of the high velocity clouds (HVC's)
arriving from the intracluster medium of the Local Group. The
underlying rotation curve is explained conventionally via the
distribution of the baryonic and dark matter components of the galaxy
in question. We are able to connect the oscillations in the outer
Galactic rotation curve with the observed major fluctuations of
star formation rate with time observed in the solar neighbourhood.
This scenario, though tested here on the two major Local Group
objects, is in principle applicable to galaxies undergoing minor
mergers with subgalactic mass gas clouds.