  We perform a Dalitz plot analysis of D^+ -> K^-pi^+pi^+ decay with the CLEO-c
data set of 572 pb^-1 of e^+e^- collisions accumulated at the psi(3770). This
corresponds to 1.6 million D^+D^- pairs from which we select 140793 candidate
events with a small background of 1.1%. We compare our results with previous
measurements using the isobar model. We modify the isobar model with improved
description of some of the contributing resonances, and get better agreement
with our data. We also consider a quasi-model-independent approach and measure
the magnitude and phase of the contributing K pi S wave in the range of
invariant masses from the threshold to the maximum in this decay. This gives an
improved descriptions of our data over the isobar model. Finally we allow for
an isospin-two pi^+pi^+ S-wave contribution, and find that adding this to both
the isobar model and quasi-model-independent approach gives the best
description of our data.
