We assess the contribution of various types of solar wind structures (coronal mass ejections (CMEs), high-speed streams, and slow solar wind) to averages of the aa geomagnetic activity index (aa) during the solar cycle. We used solar wind plasma, magnetic field, and energetic particle data to identify the flow types present in the near-Earth solar wind during 1972--1986 (encompassing the decline of solar cycle 20 and all of cycle 21). Corotating high-speed streams contribute ~70% of aa outside of solar maximum and ~30% at solar maximum (1978--1982). CME-related structures (shocks/postshock flows/ejecta) account for ~50% of aa at solar maximum and <10% outside of maximum. Slow solar wind contributes ~20% throughout the solar cycle. Our analysis provides insight into the cause of the so-called Gnevyshev Gap in geomagnetic activity, characterized by a decrease in aa near the peak of the sunspot cycle. An example of this phenomenon occurred in 1980 at the maximum of cycle 21 when aa registered a value lower than that observed at the preceding solar minimum. We attribute the 1980 depression in aa to a temporary reduction in average solar wind speed, evident in both CME and corotating stream related components, and a reduction in mean magnetic fields in all types of solar wind structure. This involvement of all solar wind structures is indicative of a global solar phenomenon, apparently related to an observed depression in the solar open magnetic flux at the time of solar magnetic field polarity reversal. Both CMEs and streams contribute to geomagnetic activity on either side of this minimum. Thus, at least for cycle 21, the Gnevyshev Gap does not reflect a transition between a period of enhanced geomagnetic activity levels due to CMEs just prior to solar maximum and a second enhancement, due to corotating streams, during the descending phase. Beyond the post Gnevyshev Gap peak, high-speed streams will eventually dominate geomagnetic activity on the decline of the cycle and may, on occasion (as in solar cycle 20), produce a late peak in average geomagnetic activity with relatively little contribution from CMEs. ¿ 2000 American Geophysical Union