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Paper No. 13-4
Presentation Time: 8:30 AM-5:30 PM
ANTHROPOGENICALLY INDUCED SINUOSITY CHANGES ON THE CHANNEL OF THE LOWER RIO GRANDE
Large-scale dam building and river diversions on the Rio Grande over the last one hundred years have reduced its discharge to 18% of pre-dam-building values and have nearly eliminated the suspended sediment load reaching the delta and Gulf of Mexico; current suspended sediment concentration delivery to the Gulf of Mexico is 1% of pre-dam-building values. The new anthropogenic conditions have undoubtedly forced the river to adopt broad changes. Here we explore how the river has adjusted the sinuosity of its channel on the delta portion of its watershed, where the river gradient is 8 cm/km and the valley length is 88.12 km. We compare pre-dam sinuosity using the 1:10,000 scale International Boundary & Water Commission map of 1912, with that of 2014, using satellite imagery from the US Department of Agriculture, National Agriculture Imagery Program, with 1-meter pixel resolution. ArcGIS software was used to geo-reference the 1912 map, create models, and precisely measure channel length on both data sets. Our findings show that in response to the new conditions the river reduced its channel length by 10 km, from 202.35 to 192.31 km, which translates to a reduction in sinuosity from 2.3 to 2.2. This seemingly small change is the river’s way to adjust its gradient to more effectively discharge its load to the Gulf of Mexico. The sinuosity change occurred through lateral migration of the meandering channel. The starting point of our measurements corresponds to the Hidalgo – Cameron County line and it therefore did not change; the ending point is the mouth of the Rio Grande and it remained fixed for the 102-year period cover in this study. Two unanticipated results of this study are, 1- the international boundary shifted as the channel migrated laterally, and as consequence the United States gained 4 km2 of land, and 2- In the delta there are portions of the channel with sinuosities of 2.5, these are among the highest reported for any channel.