From Hydraulics of Roman Aqueducts : Steep Chutes, Cascades and Dropshafts (PDF warning):

The designs of dropshaft cascade, as well as steep chute followed by dissipation basin, show that the Roman aqueduct engineers were able to design specific features to cope with steep sections. It remains unclear whether they had some understanding of the hydraulic principles, or worked by observations and trial and error.

Most aqueducts were enclosed (covered) along their entire length, limiting the possibility for gas transfer at the free surface. Thus, the downstream waters were low in dissolved oxygen content unless reoxygenation devices were installed. I suggest that dropshafts may have been introduced in place of steep chutes in order to reoxygenate the water as well as to dissipate the energy of the flow. Aeration technology is commonly used today to reoxygenate depleted waters and to enhance the water quality. I recommend that further work by archaeologists focus on the excavation and survey of chutes and dropshaft to confirm this hypothesis.