R help - Jan 2006 - Connectivity across a grid above a variable surface

Hi, 
I'm looking for ideas or packages with relevant algorithms for
calculating the connectivity across a grid, where connectivity is
defined as the minimum amount of cross-sectional area along a continuous
path.  The upper boundary of the cross-sectional area is a fixed
elevation, and the lower boundary is a gridded surface of variable
elevation.  My variable elevation surface represents the top of an
impermeable geologic layer.  I would like to represent the degree to
which a fluid could flow from one end of my grid to another, above the
surface and below the fixed level.  I don't need to derive information
about path lengths and hydraulic gradient, but if I could, that would be
a plus.  A groundwater flow model would provide the exact answer, but
I'm looking for something more approximate and faster.  

My grids are such that there are many "dead-end" flow paths, where the
bottom boundary rises to meet the top boundary and the cross-sectional
area available for flow pinches out.  In plan view, fluid can enter all
along one boundary and leave all along the opposite boundary, but flow
connectivity across the grid varies between bottom boundary scenarios.

Scott Waichler
Pacific Northwest National Laboratory
scott.waichler _at_ pnl.gov