  This letter presents a new approach using the cosmic peculiar velocity field
to characterize the morphology and size of large scale structures in the local
Universe. The algorithm developed uses the three-dimensional peculiar velocity
field to compute flow lines, or streamlines. The local Universe is then
partitioned into volumes corresponding to gravitational basins, also called
watersheds, among the different end-points of the velocity flow lines. This new
methodology is first tested on numerical cosmological simulations, used as
benchmark for the method, and then applied to the {\it Cosmic-Flows} project
observational data in order to to pay particular attention to the nearby
superclusters including ours. More extensive tests on both simulated and
observational data will be discussed in an accompanying paper.
