  The shallow shelf approximation is a better ``sliding law'' for ice sheet
modeling than those sliding laws in which basal velocity is a function of
driving stress. The shallow shelf approximation as formulated by \emph{Schoof}
[2006a] is well-suited to this use. Our new thermomechanically coupled sliding
scheme is based on a plasticity assumption about the strength of the saturated
till underlying the ice sheet in which the till yield stress is given by a
Mohr-Coulomb formula using a modeled pore water pressure. Using this scheme,
our prognostic whole ice sheet model has convincing ice streams. Driving stress
is balanced in part by membrane stresses, the model is computable at high
spatial resolution in parallel, it is stable with respect to parameter changes,
and it produces surface velocities seen in actual ice streams.
