Meteorologist Dr. Hal Needham explains the science behind a sea of many colors.
The water in Galveston is as ever-changing as the tide. Much like the weather, give it an hour and it could very well change. In this article, we take a deeper look at the many factors and the science behind Galveston's water conditions in an effort to clarify some common misconceptions.
As I admired a few oil-on-canvas landscapes at ArtWalk, something caught my attention: our Gulf Coast water color has always been changing.
I paused at a painting of a summer evening at sunset, with silver water reflecting the glow of a towering white cloud on the horizon. Other paintings featured large ships or small vessels floating on different colors of water, from emerald green to deep blue or a murky brown. And, of course, countless sunrise paintings in art galleries all over the city colored the water in various hues of yellow, orange and red.
Local landscape paintings have always depicted a wide range of water color in this part of the world. Marine and landscape painter Julius Stockfleth left us with more than 200 paintings of Texas subjects, many of which portray coastal scenes in a realist style and show a wide range of water colors. He lived in Galveston from 1885 to 1907.
Apparently, the dynamic and temperamental nature of our water’s changing color has always been a part of life along the Texas coast.
What I saw in the paintings confirmed what I have observed for years of morning and evening beach walks on Galveston Island. The water conditions and colors change rapidly here, sometimes multiple times within one walk. These realizations drew me to investigate the science behind the dynamic water conditions in this part of the world.
In its natural state, water is clear. Water surfaces act as a mirror, reflecting the landscape, or "skyscape" near the horizon. When mostly clear water reflects a blue sky, the water itself appears as a deep blue color. Bright clouds reflecting on the water often appear white, or various shades of grey.
Two of the main players that change water’s color from its clear state to a darker alternative are sediment and organic material. Organic materials, such as phytoplankton, often turn water various shades of green. Sediment often turns the water a darker color, ranging from murky black to coffee brown or lighter shades of brown or yellow.
This article explains the various sources of both sediment and organic material along the Upper Texas Coast and the complex interactions in the region from water currents and extreme weather events. A major theme of this article is that changing water colors along the Upper Texas Coast is a natural phenomenon that has occurred throughout the history of this region.
The Upper Gulf Coast observes a surprising number of blue water days. So many people are surprised to see pictures of deep blue water or to hear stories about people seeing their feet in waist-deep water in places like Galveston.
Water clarity tends to improve during consecutive days with light winds in the coastal environment. Lower wind speeds translate to calmer water, enabling much of the sediment in the water column to fall to the seafloor. Surface wind speeds like this are often localized, and even in fair weather, they can increase in a small area near thunderstorms.
On a regional scale, several factors play into a seasonal pattern that produces the greatest number of blue water days during the late summer and autumn. One reason relates to the wind direction in this region. Later in the summer, a persistent south wind often sets up in the Western Gulf of Mexico. This wind helps direct a south-to-north moving pulse of water that moves north along the Texas Coast, bringing bluer water that is more saline, according to research professor and oceanographer Piers Chapman (Chapman 2021).
Water moving "up" the coast like this blocks the push of less salty water with a higher sediment load approaching the Upper Texas Coast from Louisiana. As a result, satellite images sometimes show a sharp contrast between the blue water moving north along the Texas Coast and browner water south of Louisiana.
Autumn is also a favorable time for blue water days in the region because the amount of sediment discharged from rivers usually drops off substantially during that time of the year. By October, for example, spring storms and snowmelt are far behind us, and the heavy rains of the tropical weather season are winding down.
Most of the Mississippi River's massive watershed receives maximum precipitation in summer. Much of the Ohio River Valley and mid-Mississippi River Valley observe the wettest weather in spring, and portions of the lower Mississippi River Valley observe the wettest months in winter. But very few sites in this massive watershed observe their wettest months in autumn, except for a few sites near the Texas/ Louisiana border.
Cody Davis, a guide with Green Tide Surf Fishing, noted that the blue water of late summer and fall can last a long time if the winds remain light. He observed that blue water is also sometimes seen with an onshore wind from the southeast if the recent sediment load of the Mississippi River has been low. He added that this is most common during the autumn months.
Heavy rainfall washes soil into rivers. These rivers discharge large amounts of sediment into oceans, bays and gulfs, reducing water clarity and making the water darker. At least six major rivers discharge sediment close enough to the Upper Texas Coast to change the water color in that region. They include the Mississippi and Atchafalaya rivers in Louisiana, the Sabine (including Neches) River along the Louisiana-Texas border, and the Trinity, Brazos and Colorado rivers in Texas.
The tremendous rainstorms and floods we observe along the Gulf Coast enhance the sediment load of rivers at times. The most severe flood events cause a disproportionate amount of flood impacts in the region, including loss of life and financial losses, while also discharging enormous amounts of sediment.
Hurricane Harvey (2017) is a recent example of a storm that inflicted severe losses from days of torrential rain in Southeast Texas. Nederland, a small community near Port Arthur, observed 60.58" of inches of rain during Harvey, setting the national single-storm record for the most rain observed by a tropical cyclone (Blake and Zelinsky 2018).
In addition to the 68 lives lost (Blake and Zelinsky 2018) and an estimated $125 billion in losses (National Hurricane Center 2018), Harvey discharged a tremendous amount of sediment into the coastal waters. The storm discharged the equivalent of 18 years' worth of sediment load into Galveston Bay and pushed a plume of sediment that extended for 34 miles into the Gulf of Mexico from the mouth of Galveston Bay (Du and Clay 2019).
The Mississippi River discharges a substantial amount of sediment into the Gulf of Mexico because its watershed is so expansive, including many areas that observe large amounts of snowmelt in the spring. This watershed extends from the Rocky Mountains to the Appalachians, including nearly all the Great Plains. The combination of melting snow and heavy spring rains leads to a substantial rise in the Mississippi River every year, particularly from March until May.
The engineered landscape along the Mississippi River system has increased the amount of sediment discharged into the Gulf over time. For generations, the river overflowed its banks every spring, depositing nutrient-rich sediment along the Louisiana delta. However, the River has become highly engineered in recent times, with manmade levees keeping it within its banks during flood season. As a result, much of the sediment that used to spread out over the Delta is now ejected into the Gulf of Mexico.
This increase in discharged sediment into the Gulf of Mexico may be tempered, somewhat, by other engineered landscapes along the Mississippi River and its tributaries. For example, numerous dams along the Missouri River catch sediment while creating reservoirs, thereby reducing the amount of silt that the Missouri deposits into the Mississippi River. The big picture here is that these river systems are extensive and complex, and societal choices about how to engineer these waterways in places like Missouri and Iowa lead to environmental changes in the Gulf of Mexico, such as increased or reduced amounts of discharged sediment.
The sediment deposited along the Gulf Coast can be re-suspended when Gulf storms increase wave heights and churn up the coastal waters. Even on sunny days, the Gulf Coast often experiences persistent onshore winds that create whitecapped waves and churn up the coastal water. Localized coastal storms, and, less frequently, tropical cyclones, are other weather features that can create strong wind and rough water, re-suspending sediment that was previously deposited.
It makes sense that suspended sediment would turn the water a darker color, but why would sediment discharged from the Mississippi River impact the water color in Galveston, more than 300 miles to the west? The movement of this sediment has everything to do with prevailing water currents along the Northern Gulf Coast.
For much of the year, an east-to-west water current prevails in this region of the world. This current transports a substantial amount of sediment from rivers like the Mississippi and Atchafalaya to the Texas Coast. The map below, provided by (NOAA Ocean Explorer et al. 2017) shows the prevailing currents in the region. A red star has been added to this map to show the location of Galveston.
According to Piers Chapman, research professor of Oceanography at Texas A&M, Mississippi River sediment that is discharged into the Gulf of Mexico splits direction, with approximately 50% of it traveling east and 50% traveling west after it enters the Gulf of Mexico (Chapman 2021). The Mississippi River discharges such a high volume of water that it is the primary source of suspended sediment heading toward Texas, even though the mouth of the Mississippi is more than 125 miles east, and farther from Texas than the Atchafalaya River. Chapman estimates that when passing from east to west, south of the Louisiana coastline, approximately 70% of the sediment comes from the Mississippi and 30% from the Atchafalaya.
This NASA Earth Observatory satellite image depicts sediment plumes discharging from the Mississippi and Atchafalaya rivers in Louisiana into the northern Gulf of Mexico.
The Sabine River, on the Texas-Louisiana border, discharges less overall sediment than the Mississippi and Atchafalaya, but its position along the Texas border allows a higher percentage of its discharge to impact the Texas Coast. In March 2016, an atmospheric river of moisture, nicknamed the Maya Express, because of its origins near the Yucatan Peninsula, dumped 19 inches of rain in the Sabine basin, swelling the river to record levels (McIntosh and Lander 2017). The coffee-brown river shut down I-10 near the Texas-Louisiana border as its flood water and massive amount of sediment flowed toward the Gulf of Mexico. The sediment from this type of flood event generally finds its way to the Upper Texas Coast because of the prevailing east-to-west current.
Sometimes a thin current, just several hundreds yards wide, will flow close to the coast, while water less than one-half mile offshore is not affected much by this current. When this pattern sets up, the water close to the coast may have a darker color or even look like a shade of light brown, while the water just off shore is lighter or even blue. Arial photos of the coastal waters sometimes depict this sharp color boundary near the coastline.
Kermit the Frog once sang a song called, "It's not easy being green." Many things in nature appear green, including reptiles and amphibians, and, of course, vegetation. But what explains the science behind green water?
Sometimes phytoplankton can cause coastal waters to appear green. Phytoplankton efficiently turns the surface water green because they mostly occur near the water surface, where they are exposed to sunlight (National Oceanic and Atmospheric Administration 2021).
While these microscopic marine algae occur naturally in the environment, nutrient-rich waters, sometimes influenced by man, can enhance phytoplankton blooms, turning the water various shades of green. Runoff of fertilizers and other agricultural chemicals from the Midwest and Plains states can wash into the Mississippi River and discharge heavy loads of nutrients into the Gulf of Mexico, turning the water shades of green along the Northern Gulf Coast.
The watercolor along the Upper Texas Coast is dynamic and temperamental, often changing appearance and characteristics several times per day. The presence of sediment and organic material in the water can change the color many different shades of grey, brown, black or green. Much of the sediment and organic material in our coastal waters originates or is strongly influenced by the discharge of rivers in the region.
Bluewater days are most common when local winds die down, enabling the sediment to fall out of the water column as the water motion calms down. In the late summer or autumn, a south-to-north pulse of water often brings bluer, saltier water "up" the Texas coast from South Texas. This pulse of water can effectively block fresher water with high sediment loads approaching Texas from Louisiana.
Bluewater days are also more common in late summer or autumn because the sediment discharge of regional rivers has generally diminished by then, as the high volume of river runoff from the melting snowpack and spring storms has faded into the past by that point. However, long-time residents of this region will tell you that blue, green, brown and greywater can show up at any time of the year.
Coastal enthusiasts should find encouragement to understand that these processes are natural and have dominated the coastal environment since long before humans inhabited this region. So when we stand on Galveston Beach or the Bolivar Peninsula on a tranquil summer evening and watch pink and lavender clouds approach the coast over multi-colored water, we must remember that we are participants in a timeless portrait.
So take a little extra time on your next trip to the Upper Texas Coast to see the water in different weather conditions and different amounts of sunlight. You'll be amazed at the varied colors you see. And unlike Julius Stockfleth, you can effortlessly capture these images with a touch of your finger and post them instantly to social media.
Anderson, J.B., D.J. Wallace, A. Simms, and A.B. Rodriguez, 2016: Recycling sediments between source and sink during a eustatic cycle: Systems of late Quaternary northwestern Gulf of Mexico Basin. Earth Science Reviews, 153, 111-138.
Blake, E.S., and D.A. Zelinsky, 2018: Tropical Cyclone Report, Hurricane Harvey. National Hurricane Center, Miami, Florida, USA. 77 pp. Available on the Web at: https://www.nhc.noaa.gov/data/tcr/AL092017_Harvey.pdf.
Chapman, P., 2021: Personal communication with Piers Chapman, Research Professor at Texas A&M University. May 19, 2021.
Du, J., K. Park, T. Dellapenna, and J.M. Clay, 2019: Dramatic hydrodynamic and sedimentary responses in Galveston Bay and adjacent inner shelf to Hurricane Harvey. Science of the Total Environment, 653, 554-564.
McIntosh, J, and K. Lander, 2017: Modelling Atmospheric Rivers and the Potential for Southeast Texas Flooding: A Case Study of the Maya Express and the March 2016 Sabine River Flood. Presentation at the American Geophysical Union Fall Meeting 2016, San Francisco, California. Link: https://ui.adsabs.harvard.edu/abs/2016AGUFM.A51E0104M/abstract.
National Hurricane Center, 2018: Costliest U.S. Tropical Cyclones Tables Updated. National Hurricane Center, Miami, Florida, USA. Link: https://www.nhc.noaa.gov/news/UpdatedCostliest.pdf.
National Oceanic and Atmospheric Administration, 2021: What are phytoplankton? Link: https://oceanservice.noaa.gov/facts/phyto.html.
Where the Texas Coast begins.
Author
Dr. Hal Needham Meteorologist
