Motivated by the determination of proper motions in the M87 jet by Biretta, Zhou, & Owen, we have investigated the relationship between the dynamics of the jet in M87 and the kiloparsec-scale radio and optical structure. Our results show that relativistic effects play an important role in the appearance of shocks in relativistic jets. We argue that knot A in the M87 jet, which appears to be almost transverse to the flow, is in fact a highly oblique shock, and that the direction of its normal with respect to the flow is within about 10 degrees of the Mach angle (the angle beyond which no shock is possible). A modest pressure jump less than or similar to 5 at knot A is consistent with the observed small jet deflection for bulk Lorentz factors of the order of 3-5. We suggest that helical modes of the Kelvin-Helmholtz instability are responsible for the development of oblique shocks in the jet, provided that the medium external to the jet is no more than about 10-100 times denser than the jet. This implies that the kiloparsec-scale radio lobes are much less dense than the interstellar medium in the central regions of the M87 cooling flow. The radio jets in M87 are therefore driving high-pressure, low-density bubbles into the surrounding ISM. We show that the radii of the lobes are consistent with this interpretation and the inferred energy flux in the jet if the age of the inner lobes is similar to 10(6) yr, in agreement with that estimated by Turland. The resultant expansion of the lobes can comfortably power the excitation of the surrounding optical filaments via radiative shocks. The initial conical expansion of the M87 jet may be due to mass injection by stars along its trajectory and the thermal density of the gas within the radio lobes may be the result of a combination of mass loss from stars over the similar to 10(6) yr lifetime of this region of the radio source and the sweeping up of clouds that have condensed out of the interstellar medium. Since the knots in the M87 jet appear to be regions of transient overpressure inside an overpressured bubble, it is not necessary to invoke magnetic confinement of the jet. The fact that the inner lobes of M87 appear to be comparatively young regions immersed in a much larger and older structure, evident at low frequency, suggests that the output from the black hole in M87 fluctuates on a timescale of 10(6)-10(7) yr. Intermittency may result from the ejected radio plasma ''choking off'' the mass accretion into the nucleus via the cooling flow.
