The Past – Fluvial Geomorphology

During river trips, we often notice how the current slows down when the river flows into a lake or a bay. This slowing-down allows sediment that had been suspended in the flowing river to drop out, often forming a sandbar or a mudflat. Over time, the river may build a delta, where its main channel breaks up into multiple distributaries as it pushes its way through the accumulated sediments.  An example of this can be seen at the mouth of Mac Bayou, on the northwest side of Lake Charlotte. 

Mac Bayou Delta (Google Earth Pro)

For at least the last 65 million years, rivers have been flowing down into the Gulf of Mexico, dropping their sediment burdens, and extending the edge of the continent farther and farther out into open water.  (see Roadside Geology of Texas) Most of the land that we see in our area is part of the youngest sediment layer, which is called the Beaumont Formation.  At the level of Interstate 10, the Beaumont Formation’s surface is about 35 feet higher than today’s sea level.  On this surface, evidence of several very old channels of the Trinity River, which are much the same size as the present-day river, can still be seen.  Then, there came one or more periods when flow in the river went way up.  Bigger sediments were carried by the more powerful river during this time, and the size of the river meanders that were carved out became much larger.  Radiocarbon dating of these sediment deposits places these events at around 10,000 to 30,000 years in the past.  (see Field Trip Guidebook) 

Ice Ages Timeline (Dive and Discover)

As sea level went through several cycles of rise and fall due to the waxing and waning of the world’s glaciers, the Trinity cut out a deeper valley during periods when sea level was low, and then partially filled it back in during times when sea level was higher.  This left behind a series of smaller valleys nestled within larger ones, much like a set of Russian dolls.  The outermost of these valleys extends from about Exit 803 on Interstate 10 (Hwy 565 at Cove, Texas) to Exit 810 (Hwy 563 at Eminence, Texas).  Later on, flow in the Trinity returned to about what it is now, leaving the river to meander within the innermost valley.  

As we turn off from Highway 563 onto Lake Charlotte Road on our way to Lake Charlotte, we are up on the surface of the Beaumont Formation.  Between that intersection and Cedar Hill Park, the road descends about 15 feet onto an intermediate-level floodplain that was deposited by a larger, earlier Trinity River.  As we carry our boats from the park down to the launch site, we walk down from that intermediate-level floodplain onto a still lower floodplain, which is the one that is occupied by the modern Trinity River. 

Paddlers often notice that the “hillside” at the Cedar Hill Park launch site looks much like the one at the popular lunch spot on the south shore of Lake Miller.  The reason for this is that both of these shorelines represent a transition between the intermediate-level (ancient) floodplain and the floodplain of the present-day Trinity River.  That same arc of down-cutting that was created by an ancient river (shown by the broad green line on the topographic map) defines both the northern shore of Lake Charlotte and the southern shore of Lake Miller.  You may have also noticed that, during most water levels, the other “shorelines” of Lake Charlotte, Lake Miller, and Mud Lake are not earthen banks, but rather the front line of trees in the surrounding forest.  Only during low water does much of the forest floor there emerge as dry land, giving a chance for new cypress trees to take root and survive.  

During times when the present-day Trinity River is in flood, water fills the area enclosed by that old river bend, and sediment is deposited there, causing its depth to decrease.  Areas that were once mostly open water during such floods have progressively become separated into what is now Lake Charlotte, Lake Miller, and Mud Lake.  

Ancient Meander (Texas USGS Topo Maps)

This map from 1885 illustrates how Lake Charlotte and Lake Miller had not yet separated as of that time.  It can also be seen that Turtle Bay had not yet been cut off from Trinity Bay to form what is now Lake Anahuac.  That happened when the fan of sediment deposited by the advancing Trinity River delta eventually reached the bay’s eastern shore at the town of Anahuac, isolating what had previously been the upper end of Trinity Bay.  This could be thought of as a much larger version of what is happening at the mouth of Mac Bayou.  Over time, the advancing front of the Mac Bayou delta may reach the eastern side of Lake Charlotte, separating off the northern end of the lake.    

Lake Charlotte in 1885 (Chambers County: A Pictorial History)

The Present – Lakes, Swamps, and Marshes

“A swamp is a wetland dominated by trees and other woody plants.”  It differs from a marsh, which “can be thought of as a wetland dominated by grassland.”  (see Worldatlas) 

Due to their high resin content, cones from the bald cypress tree can float for months after they fall.  Eventually, they may become waterlogged and sink, but they can remain viable for two to three years.  (see Ecology and Management of Cypress Swamps)  Should they be deposited on exposed soil during a time of low water, they will sprout.  Only if they can grow tall enough to avoid being overwhelmed by the next period of high water will they have a chance to survive.  As a result of this ongoing process, there is a slow march of the forest into what was previously open water.  The trees also help to create their own habitat by trapping sediment faster than would happen due to the operation of physical forces alone.  Combined with the inflow of sediment from the Trinity River, this slow expansion of the forest has contributed to the separation of the three lakes, and it continues to make them smaller.  

An interesting phenomenon can be observed just to the east of Lake Miller, where the ancient Trinity River had scoured out its channel along the stretch where it was being pushed back to the west, toward the center of the river basin.  Cypress trees haven’t yet been able to colonize part of the deeper water here, leaving a ribbon of open water that is hospitable for the growth of marsh plants.  Hence, the formation of the “Marshy Lake,” which is surrounded by a cypress swamp that has become established on the floodplain’s surface adjacent to the scoured channel.  Some cypress trees have been able to spread out into the channel (sometimes referred to by paddlers as the “South Channel”) immediately to the east of Lake Miller, but it is still possible to paddle through them into the Marshy Lake at high water levels.  Otherwise, another channel, which is slightly to the north of this one, can be used to reach the Marshy Lake.  (You can see the location of both of these channels in the GPS track from our May 25 outing.)  

Old Scoured Channel (Garmin BaseCamp)

The Future – Advance of the Bottomland Forest 

The eventual outcome of all this will most likely be that the ancient meander bend containing Lake Charlotte will become completely filled in by cypress forest, with the possible exception of the old scoured channel to the east of Lake Miller.  The Swinney Marsh, which is located just two ancient river meander bends upstream from the one that contains Lake Charlotte, illustrates a late stage in this kind of progression.  

Swinney Marsh (Texas USGS Topo Maps)

The Take-away Lesson for Paddlers  

In the much longer run, the system will continue to be governed by how much water is coming down the Trinity River and by changes in sea level.  Lake Charlotte won’t be with us forever, so let’s get out there and enjoy it as much as we can!  

~ ~ ~ ~ ~

Acknowledgement and References

First of all, I want to express my debt of gratitude to the late Rufus LeBlanc, who helped me to learn about “clastic sedimentology” as it relates to Lake Charlotte. 

Aranow, S., Gregg, R.L., LeBlanc, R. and Norman, C.E., Field Trip Guidebook: Environmental Geology and Genetic Sequence Analysis of the Trinity River Valley-Delta Region Chambers and Liberty Counties, Texas, ed. Freedenberg, H., and Rieser, R.B., Houston Geological Society, 1990.  

Brandt, K., and Ewel, K.C., Ecology and Management of Cypress Swamps: A Review, Bulletin 252, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, 1989. 

Dive and Discover: Expeditions to the Seafloor, Woods Hole Oceanographic Institution, https://divediscover.whoi.edu/ice-ages/ , accessed June 2019.

Garmin BaseCamp Version 4.7.0, Garmin Ltd., https://www.garmin.com/en-US/shop/downloads/basecamp , 2008-2018.  

Google Earth Pro, https://www.google.com/earth, accessed June 2019. 

Henson, M. and Ladd, K., Chambers County: A Pictorial History, The Donning Company, Norfolk, 1988.  

Spearling, D., Roadside Geology of Texas, Mountain Press Publishing Company, Missoula, 1991.  

Texas Seamless USGS Topographic Maps on CD-ROM, National Geographic Holdings, Inc., 2001. 

Worldatlas, https://www.worldatlas.com/articles/what-is-the-difference-between-marshes-and-swamps.html, accessed June 2019. 

---

The author, Tom Douglas