  For certain families of fluid flow, a new conserved quantity --
stream-helicity -- has been established.Using examples of linked and knotted
streamtubes, it has been shown that stream-helicity does, in certain cases,
entertain itself with a very precise topological meaning viz, measure of the
degree of knottedness or linkage of streamtubes.As a consequence,
stream-helicity emerges as a robust topological invariant.
