It defines what is meant by the South bank as being the high water mark. However, I find compelling the arguments of the dissenting opinion, that the boundary should be the low water mark.
Primarily because the Treaty of 1819 clearly intended to let the people on both sides of the river full access and use. Using the low water mark accomplishes this.
Using the High-water mark leaves a strip of ground that would be foreign territory to the occupants of the remaining Spanish territory, which is not likely. Also the fact that it was necessary to state in the treaty that the islands were USA territory.
There would be no need to do this if the high water mark was intended to be the boundary. Also, this finding means that there is no set boundary, as it is subject to change of the river by accretion.
Using the low water mark would make for a more set/permanent boundary. JMHO Supreme Court erred in deciding to use the high water mark. I am not a lawyer, but I do think logically.
The decision to use the high-water mark essentially robs the Texas landowners of there riparian rights, since for example, their cattle can only access the river by crossing land that is now claimed to be held in trust for the Indians, and under BLN control.
Using the low water mark, would give them direct access to the river, at all times.
The boundary issues really came up when oil was discovered along the river. Oklahoma and USA tried to extend the boundary as far south as possible. Texas tried to extend it's claim North to the medial point of the river.
http://www.law.cornell.edu/supremecourt/text/260/606 The Texas/Oklahoma boundary compact now speaks of the vegetation line on the South bank. I'm not sure that is going to be of any more benefit to the people living on the Texas side of the Red River. It may be that Texas is once again going to have to go to court in order to preserve the property rights of their citizens. With the current court, I am not sure that's gonna work.