The hydrodynamic forces produced by breaking waves make the rocky intertidal zone one of the most physical stressful environments on Earth. Although the classical in-line hydrodynamic forces are well studied and characterized, an undescribed force -- known as the impingement force -- may be the largest hydrodynamic force in the intertidal zone. Impingement is characterized by a sharp, transient spike in force at the instant of wave arrival, and very few measurements of it exist. I measured impingement in the laboratory, using a gravity-driven water cannon to simulate waves and tested which variables affect impingement magnitude. I show that impingement is likely drag, rather than an undescribed hydrodynamic force; it likely occurs due to brief increases in flow velocity at the front of a wave. To assess the frequency and magnitudes of impingement events in the field, I built a force transducer to record high-frequency forces in the intertidal zone. Impingement events occur in 2-7% of waves in the field, but surprisingly, the average magnitude of impingement events is lower than the average maximum force produced by all recorded waves in this study. This suggests that impingement is not the largest hydrodynamic risk to organisms, and future work measuring flows in the intertidal zone should focus on identifying large transient forces regardless of where they occur in a wave. Using these measurements, I explore the likely effects of brief forces on intertidal organisms in two ways: first, I model a limpet as a mechanical spring-mass-damper system and show that its foot has the capacity to reduce the effective force on the animal. I also predict dislodgment rates of three species of gastropod using the maximum forces recorded in my field study. At my site, snails are very susceptible to wave-induced dislodgment. This dissertation illustrates the necessity of recording high-frequency flows when studying intertidal hydrodynamics.