Rivers change as time goes on, natural erosion, severe weather events and human intervention can all contribute to a changing landscape. The Navasota River is no exception to this. Flowing 125 miles from Mount Calm in East Texas, it eventually joins the Brazos River and is dammed in six places to form manmade lakes.
These natural and unnatural changes can have consequences: degraded water quality, altered or destroyed habitats, flash-flooding and more.
Research led by Texas Water Resource Institute Research Specialist Ed Rhodes, titled , A changing river: Long-term changes of sinuosity and land cover in the Navasota River Watershed, Texas, along with Jackson State University assistant professor Rocky Talchabhadel, Ph.D., and former TWRI Graduate Research Assistant Taylor Jordan, uses historical images and records to better understand the changes that the Navasota River has undergone in the last 50 years and how that has potentially caused more flooding in the southern portion of the watershed.
How the river changes
This research is one part of many different ongoing projects related to flooding and disaster-related research from the Institute for a Disaster Resilient Texas at Texas A&M Galveston and was funded by the Texas A&M University System Chancellor’s Office.
Rhodes became part of this research project because of his experience in using remote sensing of historical imagery in the Ogallala Aquifer Program and other projects. Using historical images of the Navasota River from 1972 to present date, Rhodes compared the path of the river over the decades to see if there were changes to the river’s path, and if so, how severe.
“I wanted to look at the velocity or meandering of the river. Because the more bends in the river, the more volume of water it can hold,” Rhodes said. “We wanted to see if the river had been getting more winding or more straight.”
Landowners near the Navasota River with land in the southern portion of the watershed were concerned that it seemed as if their property was flooding at a greater frequency than in the decades before, he said.
To find those answers, Rhodes traced the portion of the river from Robertson county to down along the eastern edge of Brazos County to where it meets the Brazos River in Washington County.
“We tried to use computer software to extract the shape of the river, but with the historical imagery it wasn’t working, so I hand traced it,” Rhodes said. “We got the images from the USDA, they have a massive warehouse of historical photos and we georeferenced the photos.”
Georeferencing means that photos do not come with exact coordinates, that makes it too easy, he jokes. The team had to take the historical images and compare them with current pictures of the river, finding where landmarks like intersections and buildings that still exist match to stitch together the entire river boundary. After that, the process turned mathematical.
“Once the pictures were stitched together, I traced the river boundary,” Rhodes said. “I basically created a polygon of the river and then used a tool in ArcGIS that created a centerline, the center point of the river.”
Measuring that line, along with the ratio of how straight versus meandering the river was, allowed Rhodes to get a general idea of how much more or less volume the river was able to hold at the times the images were taken.
More curves hold more water and also slow water down, typically keeping water levels lower as the water in the lower portion of the watershed has time to flow out. The straighter the river, the faster the water moves and the more chance there is for flooding, especially downstream, he explains.
“Overall, the whole stream didn’t change all that much,” Rhodes said. “But if you broke it down into segments, there were some segments that got longer, some got shorter, one segment in the upper part of the watershed shortened significantly.”
Flooding factors
Off-take channels were another factor looked at as a contributor to downstream flooding. These channels act as stress valves he said, these are sections where the river splits, most of the time eventually flowing back into the main channel.
“In 1972 up until 1995, the number of off-take channels was increasing,” Rhodes said. “Since then, they’ve gone down. The lack of those could be contributing to flooding.”
Historical records have also shown a gradual increase of rainfall overtime, he mentions. Combined with urban development in communities like Bryan-College Station within the watershed, all of these factors are flood factors.
While not all of Bryan-College Station is in the Navasota watershed, the portion that is, including part of the Texas A&M campus, drains into creeks that lead to the Navasota.
“All of the urban development in the Bryan-College Station area has led to a huge increase in the amount of impervious cover, roads, buildings, medians; there’s been a 39% increase in impervious cover,” Rhodes said.
There are other potential factors that were not studied in this research, Rhodes mentions. Things like dams potentially trapping sentiments, keeping the river from a more natural change process and how the manmade lakes in the upper watershed could be affecting water levels.
“We didn’t study that, but we can’t completely rule out the effects of the dam,” Rhodes said. “There are a lot of other combinations of things that are going on in the watershed.”
Looking to the future
Rhodes and his team’s research have helped better understand how the Navasota River has changed over the last 50 years, though there are still uncertainties.
“With these historical images you can measure the stream or the river, but we can’t measure how deep it was,” Rhodes said. “We don’t actually know the total volume that the river did, or can, hold.”
Going forward, he said, long term studies would need to collect physical measurements to more accurately record the volume of the river.
An interesting find when looking at the historical images, he said, was seeing that the Navasota and Brazos rivers were slowly moving towards each other and will eventually connect. Rhodes said this will likely make the Navasota River shorter and potentially cause more flooding.
Research like this, Rhodes hopes, encourages people to examine all facets of issues to better understand the how and why of problems.
“Think about the different angles when looking at an issue,” Rhodes said. “There’s not always a single cause, there’s an accumulation effect because you have changes in land use, in precipitation and the just natural changes in the flow of the river. Don’t be quick to decide that it’s just one thing when there could be multiple interactions of things going on.”
