276-3 Why are the Missions so Close Together? River Incision as a Constraint on Location of Early Settlement at San Antonio

Session: Natural and Urban Channel Responses to Disturbance, Management, and Restoration

Presenting Author:

Author:

Abstract:

                  Five Missions and a civilian settlement were located along the spring-fed upper San Antonio River in the 1700s, using acequias fed from the river to supply water. Only the headwaters from the spring to Mission Espada (18.5 valley km) are suitable for mission irrigation, as the valley there is wide and gently sloping, and the river is not deeply entrenched (incised) and inaccessible as it is to the south.

                  Topographic profiles show that the river is not incised near the springs. It lies in an open valley 4-6 m deep from downtown as far as Mission Espada. To the south of that point, it is sharply incised within 5 km to a depth of 15 m at Blue Wing crossing. Incision continues past the Medina River confluence (at 10.3 km) to the Bexar/Wilson County line (24 km). Incision depth then decreases downriver to 9-11 m, and the valley becomes more open.

                  The Medina River is incised to 15 m depth for 36 km upriver of the confluence as far as Macdona. Incision then declines westward (upriver) to 7 m at Lacoste where the valley is more open. Side creeks are incised as they approach the main streams.

                  Mapping and profiles show that the rivers are bordered by young terraces within the active floodplain. The rivers and their young terraces are incised into the broad Applewhite terrace (Holocene 12-5 ka) in a trench that is typically 150-600 m wide, with side slopes up to 30%). The Applewhite terrace acts as a high floodplain that is covered with muddy water during 100-year floods. The profiles show that levees up to 1.5 m high are developed on the terrace, grading away from the river; several Yazoo streams are located within the back swale.

                  The cause for late Holocene incision in southern Bexar County is not known. It does not appear to be controlled by bedrock lithology; however, deep incision ends downriver at the Carrizo sandstone outcrop.  By the time of historic settlement, the upper limit of incision had not yet retreated northward along the San Antonio River beyond Mission Espada, which allowed for the historic settlement of the river north of that point.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5615

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.