  We present multi-line and continuum observations of the circumstellar
environment within 10^4 AU of a sample of protostars to investigate how the
effects of outflows on their immediate environment changes over time. 12CO(1-0)
emission probes the high-velocity molecular outflows near the protostars and
demonstrate that the outflow opening angle widens as the nascent star evolves.
Maps of the 13CO(1-0) and HCO+(1-0) outflow emission show that protostellar
winds erode the circumstellar envelope through the entrainment of the outer
envelope gas. The spatial and velocity distribution of the dense circumstellar
envelope, as well as its mass, is traced by the C18O(1-0) emission and also
displays evolutionary changes. We show that outflows are largely responsible
for these changes, and propose an empirical model for the evolution of
outflow-envelope interactions. In addition, some of the outflows in our sample
appear to affect the chemical composition of the surrounding environment,
enhancing the HCO+ abundance. Overall, our results confirm that outflows play a
major role in the star formation process through their strong physical and
chemical impacts on the environments of the young protostars.
