Otherwise, the river flows SE. Can you enter the US, from Mexico, on a boat?
For most of the Rio Grande, you can walk across by wading over. Hence the old term “wetback” for illegal Mexicans crossing the border.
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Yep on a boat, float tubes, wading, etc, as some places are wide but not necessarily deep.
There have been times when the water doesn't flow enough to reach the Gulf of Mexico at Brownsville.