Warp is a modifier that uses its sub object to deform its targets points. For example, using a sphere as a sub object one can deform a planar mesh to spherical mesh.
To use the tool, multi select one or more target objects (nurbs curves, particles, freeform surfaces etc.) and one deformation geometry object (say an analytic sphere) and enter two points through the view window to define the bounding box for the warp object, in this order.
The warp object defines the following options for controlling the warp deformation:
If set only the points inside the objects bounding box will be deformed.
Set to project all points onto the surface of the deformation geometry.
Disable extrapolation of the deformation geometry for the points that don't fit inside the bounding box of the warp object.
Strenght of the warp effect.
To create a waterfall of particles:
1. Create 1D particles and animate the translate attribute on object's 'X' axis. So you have a flock of particles moving horizontally in some very boring way.
2. Create a bended NURBS mesh representing the shape of the waterfall.
3. Multi-select the particle object and nurbs mesh, activate the Warp tool and and enter two points in the view window to define bounding box for the warp deformer.
4. Play the animation and the particle object will bend along the surface of the NURBS object.