Throne of Thorns
The closest thing the River Kingdoms have to a unified set of laws is the code of justice known as the Six River Freedoms. They are normally ranked in ascending order of importance. Thus free speech is not considered as important as freedom from slavery, for example.
Say What You Will, I Live Free
This is the freedom of speech (and not to be confused with freedom from the consequences of your words). Criticism of government is common in the kingdoms, and lords of the kingdoms who are lenient with loose tongues are likely to live longer.
Bards take special advantage of this freedom, and spells like silence are viewed in a harsh negative light.
Making an oath is a huge deal for the people of the River Kingdom (where trust is hard to come by, and earned in blood), and this freedom grants the people of the kingdoms leave to persecute anyone person that breaks an oath. Riverfolk who take on oaths usually keep them, or die trying.
This freedom can have some constricting side effects on traders and businessmen, who are loath to make strict commitments or promises.
Walk Any Road, Float Any River
This freedom prevents Rulers of the River Kingdoms from blocking travel over land and water, including charging tolls for passage.
The fate of the town of Heibarr is sometimes uses as a salutary tale of what can happen if the third freedom is disrupted.
Courts Are For Kings
One of the most basic freedoms of the River Kingdoms, this one holds that all laws within the river kingdom are flexible, and that rulers of a kingdom may do as they wish. Visitors to a river kingdom — be they king or commoner — are bound by the (often arbitrary) laws of that kingdom. Consequently rulers of the different kingdoms infrequently visit each other, and instead rely on liasons and intermediaries. The exception to this is the annual Outlaw Council.
Slavery is an Abomination
Escaped slaves are an important fact of life in the River Kingdom. A slave that escapes to the River Kingdoms is considered truly free. By some estimates, more than a third of all people living in the River Kingdoms are either escaped slaves or children of escaped slaves. Thousands of slaves make their way to the River Kingdoms annually and fiercely defend their newfound freedoms.
You Have What You Hold
This freedom draws the moral distinction between stealing and robbery. In the River Kingdoms, it is more preferable to face your robber, to be allowed the opportunity to resist (and perhaps to repossess!). It is acceptable (and perhaps worthy of praise) to take what you want by force.