Recent archaeological research at the urban settlements of Lukurmata and Pajchiri on the southern shores of Lake Titicaca, Bolivia has revealed that open channel flow aqueducts were critical elements in the technological repertoire of Tiwanaku hydraulic engineers. Six stone-faced and rubble-filled aqueducts directly linked to partially modified and canalized natural <i>quebradas</i> were identified, mapped, and dated by ceramic and structural associations to the Tiwanaku IV-V phases (ca. 400-1100) at these two sites. A 50 m. segment of the smallest aqueduct at Lukurmata was completely excavated to its base, while the intentionally shaped channel of a massive aqueduct at Pajchiri was surface profiled and mapped at intervals of 3 m. over a nearly 90 m. long, mid-structure segment. The modern theory of the hydraulics of open channel flows is applied to the analysis of aqueduct channel morphology, slope, structural characteristics, and design intent. The analysis concludes that these aqueducts functioned primarily as drainage structures which served to reduce the infiltration of surface water into the groundwater reservoir, and not to irrigate closely associated raised field, agricultural complexes.