By Eugene Fernandez, Special to the Herald
To more completely understand the geography of the Rio Grande and its delta region, inclusive of its resacas (lagoons), one must travel back in time about 28 million years before the present.
This was a period of continuous mountain building in the Western United States, and it formed the upheaval that would become the San Luis Hills and the base range of the San Juan and the Sangre de Cristo Mountains of Southern Colorado.
This action created a corridor that was to become the Rio Grande River Basin. This formative era was followed by a time of great volcanic activity, which drew to a close between 17 million and 16 million years ago, leaving a landscape that would provide a foundation for what exists today.
The lands beneath the snowladen peaks of that range became terraced receptors for seasonal run-off of ice and snow. The water underwent a process of breaking through plateau rims to form gorges that carved paths, eventually leading seaward to what would become the Gulf of Mexico.
The first trickle of Colorado water reached the gulf between 1.6 million years and approximately 600,000 years ago. It was from that point in time that a delta began to form.
The level of the gulf as it appears today was attained approximately 6,000 years ago. The delta began to be constructed when sea level was about 30 feet lower than it is today. The earlier, more primitive periods witnessed a radical water run-off due to the steepness of the decline of the continental shelf for that time. As the sub-aquatic base of the delta grew, the alluvial deposit was more broadly distributed and a fan delta began to form.
The delta extension reached a maximum outward protrusion about 3,000 years ago. The greater mass of the fan base has “transgressed,” or eroded over the past 2,000 years as a result of lesser alluvial activity along its path and losses from water-born currents circulating in the gulf.
Internally, in the evolution of a delta there is eventually a phenomenon known as “push-back” upon its own deposited mass.
Literally, a point is reached whereby there is an attempt to push water uphill as the delta base has grown in mass and serves as a stopper to the flow that is attempting to converge with it. It is at this point that you have the creation of “distributary channels,” a nascent form of what evolved as resacas.
Bear in mind that there has always been only one primary river channel at a time on the Rio Grande. Though it has frequently altered course, the main channel remained clearly definable. The overflow corridors that serve as escape valves for a river at full flood stage are what we refer to as distributary channels, to become “resacas” in the local jargon. The historical common source for each of these channels (river port) is the Rio Grande, though the umbilical has been hidden by eons of silt deposit and even the hand of man. From aerial views, it is somewhat difficult to discern the historical main pathway of these resacas because there has been no consistent route in their meanderings.
From one flood to another, silt deposits would build up intermittently and send the flow off in a new wayward direction.
Evidence of this is written upon the land in the form of soil discoloration that is seen in plowed fields from aerial views. What we see upon the landscape at the present day is but a brief, final footprint that has settled after the last taming of the river, which largely came into being after the building of the lower river dams (1952).
Although it is not considered among the primary cluster of deltaforming resacas that spread out over the delta from south of Harlingen onward, the Arroyo Colorado indeed establishes the first departure from the dedicated Lower Rio Grande channel. This grouping makes an obvious departure to the east-southeast commencing with the Arroyo Colorado diversion. The Arroyo’s original connection to the source river would have been at about the area where Anzalduas Dam is now, just above the presentday town of Granjeno. A study of the topography of the Rio Grande Valley reveals that there would be somewhat of a free-fall of riverborne water from about Rio Grande City, which has a height of 174 feet above sea level, then downward to McAllen at 122 feet, Granjeno at 108, and Weslaco at 79 feet above sea level. Mercedes sees the last of the 100-foot elevations, and then you descend upon the Texas Coastal Plain — prime geography for delta making.
Eugene Fernandez is director of the South Texas Center for Historical & Genealogical Research-501(c)(3) Historical Research Management Foundation.