Nebraska v. Iowa was an 1892 Supreme Court case between the states of Nebraska and Iowa about the status of Carter Lake, Iowa. During 1877, a flood in the Missouri River had shifted the main course of the river, shortening it and leaving Carter Lake on the Nebraska size of the river, with the former river course turning into a relatively shallow oxbow lake. The question that placed this before the Supreme Court was about the doctrine of rivers shifting, as how it related to both private and public property.
The traditional doctrine was that the shifting of a river's course, and the loss of land on one side and the accretion, added or subtracted property or territory from one party to another, and that the river was the boundary, no matter how its course shifted. But in this ruling, the court, in an opinion authored by David Brewer, said that the doctrine of accretion did not apply here, but rather the doctrine of avulsion, because rather than shifting soil from one side to another, the river has changed its channel, in which case the old channel, empty of water, is still the border.
This is actually something that can be explained with common sense, and has a very common sense application. The difference between the gradual shifting of soil from one side to another of a river is a vastly different process from a sudden change in a river course. And while it would be technically taxing to measure scraps of alluvial soil on one side of a river and try to give them to the owner on the other side, when a river changes its course suddenly, large areas of established land with people and buildings are now in a different place. So the practical reasons for this decision are fairly obvious to explain. (Although, of course, there is always some rules lawyering to be done: what if there is not an obvious answer or expert consensus about whether a river changed by avulsion or accretion, but that is how lawyers keep in business.).
What is more interesting to me than the obvious answer is how Brewer and the Court reached it, and what that says about the Supreme Court and American law in general. Because Brewer discussed many things in his opinion, including English common law, the law of Mexico and Spain, and even the law of river boundaries formulated by the Roman Emperor Justinian I. He also talked about the hydrographic differences of the Missouri River from other rivers, the yearly floods, the geological question of how far down the true ground, as opposed to the alluvial deposits, were, and the question of whether dirt dissolved in water can be said to be traceable, or just forms an amorphous body of possibilities that can appear and disappear in any place. Its quite a lot of legal writing about dirt.
There is a legal school of thought based around the idea of strict interpretation of the constitution, called constructionism, which is also sometimes coupled with the idea of Founder's Intent. The constitution says what it says, and any issue not directly addressed in the constitution is not up for the Supreme Court to decide. And, when faced with this doctrine, what should the Supreme Court do? What answers does the United States Constitution give us? Where does the Constitution mention dirt, mention the difference between loam and sand, and mention the difference between accretion of soil and changes of channels? What did the Founding Fathers think about shifting rivers and towns moving across state lines? From a strict constructionist point of view, the United States Supreme Court isn't meant to quote Roman Emperors or Andres Bello, or to get its hands dirty trying to decide how suddenly the bank tumbled into the river, and where that soil might have gone. What they are supposed to do in this case isn't really described by strict constructionists, because the constitution gives them the duty of solving disputes between the states, but apparently would give them no tools to do so with.
I guess the point here is that long before the buzzword of "activist judges", it was natural for the Supreme Court to come up with solutions using a combination of Common Law, International Law and common sense, to deal with matters as prosaic as a shifting river course.
The case and opinion may be read here: