  A systematic construction of St\"{a}ckel systems in separated coordinates and
its relation to bi-Hamiltonian formalism are considered. A general form of
related hydrodynamic systems, integrable by the Hamilton-Jacobi method, is
derived. One Casimir bi-Hamiltonian case is studed in details and in this case,
a systematic construction of related hydrodynamic systems in arbitrary
coordinates is presented, using a cofactor method and soliton symmetry
constraints.
