This San Antonio River study will walk you through determining the age of a this river.
The San Antonio River has a rather unique geomorphology in that it doesn’t have the usual highland headwaters source, instead it is fed from a group of springs. If course during the rainy season or rains of any season it is also fed from a series of creeks that increase its flow substantially from the spring fed flow that is its source for most of the year.
San Antonio Springs and San Pedro Springs are the main sources of springs that originate the San Antonio River. San Pedro Springs is a series of 10 springs that come up from the edge of a Limestone fault in San Antonio near San Pedro Avenue and N Flores St.
San Antonio Springs flows also from near Broadway and Hildebrand Avenue originally a fountain spring it is much less so since the drilling of the Edwards Well.
Nevertheless this is an over view of the headwaters of the San Antonio River.
For the purposes of this earthcache study in the age of the River we will evaluate where it lies In the age of a river.
Even though the beginning of the river is a mere 31 miles from this location the river has transformed itself as it flows across the land from there to here on its 242 mile length.
To determine any river's age, geologists primarily look at its landform characteristics like the steepness of its gradient, the presence of meanders, the size of its floodplain, the type of sediment it carries, and the overall shape of its valley, with older rivers typically having a flatter gradient, large meanders, wide floodplains, and a well-developed valley compared to young, steeper rivers.
The very interesting part of this study is that you will need to adjust your perception of time. In this study we arent evaluating the age compared to date. We are evalluating the age based on where the river is in it length. Does this seem a bit weird... well yes it is. Its like an evolution of a river so to speak.
A young river, from a geological perspective, is typically in its early stages of development, characterized by steep gradients and high-energy flow. These rivers are often found in mountainous or upland areas where the land is still being uplifted. The flow of water is fast and turbulent as it moves through narrow valleys, and the riverbed is usually rocky or uneven. The erosional power of the river is strong during this stage, as the water cuts deeply into the landscape, carving V-shaped valleys and often creating waterfalls or rapids. The water’s velocity is high enough to transport large sediments, such as rocks and boulders, downstream. In the instance of the San Antonio river, the headwaters are at a spring and not the usual mountain or uplands area. Many smaller tributaries add to this river above this part of the riverbed of the San Antonio River. As mentioned in the beginning of this that this river is unique in its source this still does not eliminate if from the age evaluation as it flows south towards where it joins the Guadelupe River a mere few miles before reaching the San Antonio Bay and the Gulf of America.
As the San Antonio river continues its journey, it tends to erode the land more vertically than horizontally. The water cuts downward into the bedrock, deepening the river channel. This process is called downcutting and contributes to the formation of features like canyons and gorges. During this stage, the river does not yet exhibit much lateral meandering, and the floodplain, if present, is relatively small. The primary focus of the river at this point is incision into the landscape rather than widening or meandering.
Over time, as a river reaches older stages, it will begin to slow down and take on characteristics of a more mature river. But in its youth, it is a dynamic agent of erosion, capable of transforming the landscape by cutting through bedrock, carrying large sediments, and creating dramatic geological features. It is also in this phase that the river is most likely to be influenced by tectonic forces, as the ongoing uplift of land continues to drive the river's erosion and incision patterns.
A middle-aged river, often referred to as a mature river, is characterized by a more moderate gradient and a wider, flatter valley. By this stage, the river has slowed down, and the erosional focus shifts from vertical downcutting to lateral erosion, meaning the river starts to meander across its floodplain. This occurs because the land has become less steep, allowing the river to erode the banks more horizontally. The riverbed is typically made up of smaller sediments, like gravel and sand, which have been transported downstream over time. The moderate slope and energy of the river allow it to develop a more meandering course, forming oxbow lakes, natural levees, and broader floodplains. This shift happens due to a combination of factors, including the reduction in the energy of the flow and the relative maturity of the surrounding landscape.
Your mission today should you choose to accept it is to seek the viewpoint of the San Antonio River at the access point where kayaks are launched and retrieved. Please send these answers to me prior to logging a find. Group answers are welcome. As you view the river answer these 3 simple questions:
1. In your opinion what is the age of the river here?
2. Describe the banks and the flow at the time you are viewing it. How high are the top of the banks to the surface flow of the river?
3. Please share where you have seen a river, any river anywhere that was a diffferent age than this one here. What was the name of the river and the age and why do you think it is a different age than this one?
Please share a photo of you at the top of the entrance to the ramp down to the river.
Sources
https://www.edwardsaquifer.net/sariver.html
https://www.tshaonline.org/handbook/entries/san-antonio-river