Thanks to drought, fast-growing weeds, and overuse of water by agriculture and cities, the Rio Grande River is running so low that it doesn’t even reach the Gulf of Mexico anymore. Enviros are concerned about the loss of the estuary, where saltwater mixes with freshwater to create ideal conditions for young shrimp and other marine life. They are calling for big changes in international water-use plans to protect the river, which marks the boundary between Texas and Mexico. Paul Montagna, a University of Texas marine biology professor, said, “It’s become more like a stagnant lake than a river. Any organisms that need to use this as a nursery can’t get out.”