Arroyo Colorado in Texas at Harlingen
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Welcome to Arroyo Colorado Park in Harlingen, Texas (Cameron County)! There is a parking lot, and park hours are sunrise to sunset. At the coordinates there is a remarkable geological feature.
This is not your typical geocache; it is an EarthCache. Instead of looking for a “cache container” you will be brought to an overlook of a geological feature.
ABOUT THIS LOCATION This EarthCache is in Arroyo Park along East New Hampshire Street in Harlingen, Texas (Cameron County). Feel free to park in the lot.
GEOLOGICAL INFORMATION The land of a large region of southeast Texas with its rich farmland, many urban communities, and numerous parks has a surface that is very flat in appearance. This geological feature, the Arroyo Colorado, is a major natural exception to that continuously horizontal terrain. Arroyo as defined in the Random House Webster’s College Dictionary is: “(chiefly in southwest U.S.) a small steep-sided watercourse or gulch with a nearly flat floor; usu. dry except after heavy rains.” In this case the watercourse had been what is called a distributary. From the same source, a distributary is: “an outflowing branch of a stream or river, typically found in a delta (as opposed to a tributary).”
For a very long time the ancient Rio Grande and its distributary transported sand and silt to this vast area forming a growing delta region. Harlingen is on what is called the Coastal Plain of Texas which earlier was a delta. This Arroyo Colorado Delta was formed by deposits left by the distributary that flowed in this arroyo. Deposition and thus growth of the delta, referred to as progradation, was not a continuous process, however. There were also periods of erosion of the delta, referred to as transgression. These main periods of deposition and erosion were dependent upon what was occurring during the Ice Age/Pleistocene Period. During glacial advance there was erosion, while during an interglacial period there was deposition/growth of the delta. The status of glacial activity determined the status of delta activity due to the difference in sea level.
During the last major glacial advance the headward erosion of the arroyo would start and in time it captured part of the flow of the Rio Grande. The glaciers held an enormous amount of water in the form of ice causing the sea level to be about 400 feet lower than the current level, and as a result there was erosion/transgression of the delta instead of deposition/progradation. The amount of precipitation was much greater then, also. Between 15,000 and 10,000 years ago (Before Present) the melting glaciers began to raise the sea level. Then 10,000 to 7,000 years B.P. the growth of the delta, progradation, began once more. By 4,500 years B.P. sea level was near its present level. The amount of sediment decreased, however, and thus also delta progradation. Since that time the coastal zone has acquired its present appearance. For example, the Arroyo Colorado was no longer an active distributary of the Rio Grande. Only in time of floods and great storms was/is it actively transporting huge volumes of water to the Laguna Madre from the Rio Grande's northern flood basin.
How was this Arroyo Colorado created? This arroyo was formed by headward erosion. Its stream is the only major headward eroding stream in this entire region; the others are very minor. The headward erosion of the Arroyo Colorado started when sea level began to lower during the last late Wisconsin glaciation, cutting into late Pleistocene sandy, fluvial deposits. Later, headward erosion to Rio Hondo allowed the stream to pirate runoff from the Rio Grande’s northern floodplain. Due to continued headward erosion, the Arroyo Colorado extends today many miles west of Rio Hondo. This view is just a glimpse of its total 90-mile length. It may contain an abundance of water from hurricanes and storms. Generally though, the perennial water flowing in the arroyo here is from sources such as agricultural irrigation return flow, municipal discharge/urban runoff, and base flows from shallow groundwater.
EARTHCACHE INFORMATION To receive credit for this EarthCache please do the following: 1. Photos are optional, but are appreciated, please send a photo of yourself/group and GPS with Arroyo Colorado in the background. 2. Briefly describe the stream water level relative to the trail bridge at your coordinates. 3. Is the water flow right to left or left to right, if it were a tributary? Briefly, the difference between a tributary and a distributary is _____________. 4. From your observation of the Arroyo Colorado at the coordinates, what part of the progradation/transgressive cycle do you think it represents and why? 5. The arroyo was formed by headward erosion, but what are some of the conditions/reasons that caused the arroyo to have little or lots of water in it during your present EarthCache visit? You do NOT need to wait for confirmation from me before logging your find. Please do not make any reference to these answers in your log.
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