We point out that the geometry of connected totally geodesic compact null hypersurfaces in Lorentzian manifolds is only slightly more specialized than that of Riemannian flows over compact manifolds, the latter mathematical theory having been much studied in the context of foliation theory since the work by Reinhart (1959). We are then able to import results on Riemannian flows to the horizon case, so obtaining theorems on the topology of compact horizons.