The four nations in the world of Avatar: The Last Airbender differ a great deal based on their geography, socio-political structures, clothing, customs, and more. Even within a single nation there may be some diversity, and the Water Tribe in particular is rather fractured. Split into North and South, no one would ever confuse the two cultures.
These sister Water Tribes came closer together during Legend of Korra under the leadership of Unalaq, Korra's own uncle. During Avatar Aang's time, however, the North and South were quite distinct. In fact, the Northern Water Tribe had disdain for its Southern cousin and rarely lent it aid. How did they become so separated?
The Decline Of The Southern Water Tribe
Brief flashbacks in Avatar showed that in decades past, the Northern and Southern Water Tribes were roughly equal in terms of population, size, military strength and political influence, but that changed over time. Once the Hundred-Year War was launched and the Air Nomads were wiped out, the Earth Kingdom and Water Tribe were the Fire Nation's next targets, and the South suffered greatly. Over time, Fire Nation units and privateers such as the Southern Raiders chipped away at the South, and Waterbenders were captured one by one until the last one, Hama, was finally taken.
With all Waterbenders gone, the Southern Water Tribe was practically pacified. Only conventional non-bender soldiers remained, and they posed little threat to the Fire Nation. By Katara and Sokka's time, the Southern Water Tribe was little more than a large outpost, shrinking even further when all combat-ready men departed in a small flotilla to raid the Fire Nation's navy across the world. Thus, Sokka and his grandmother became the tribe's leadership figures. They then faced another challenge when Prince Zuko arrived to capture Avatar Aang, who had just been released from his iceberg shell.
In times past, the Southern Water Tribe was a fairly liberal and relaxed place to be, especially compared to the rigid Earth Kingdom and the tradition-bound Northern Water Tribe. In the South, women are respected not just as homemakers, but also as leaders and fighters, and Hama fought ferociously until the Southern Raiders captured her. Girls of the South are free to marry whomever they choose, an unusual degree of freedom in this era. During Katara's time, there's little to show for all this. But by Korra's time the Southern Water Tribe is back to full size, and it can be presumed that this realm continued to serve as an example of liberal social policies at work.
The Traditions & Walls Of The Northern Water Tribe
By contrast, Avatar's Northern Water Tribe is almost like an icy version of the great Ba Sing Se, a safe but rigid place bound by tradition, rules and hierarchy. Sokka and Katara had to get used to this when they visited, and in their eyes, the Northerners paid more than a few prices for their culture's safety and security. In the North's favor is its tough defenses, such as the endless walls and sheets of ice that benders can morph into barriers to keep the Fire Nation's fleets away. Large numbers of Waterbenders can wash away hostile forces with great waves, and they can even form a water elevator to only allow friendly ships in and out of the city's walls. Combined with the region's remoteness and cold climate, it all keeps the Northern Water Tribe quite safe from the Fire Nation -- despite a lack of industrialization.
The Northern Water Tribe has the benefits of a large population, durable defenses and an orderly society, though there are prices to pay for this -- mainly social and personal. This tribe is stable and secure, but girls and women face customs and traditions that dictate their entire lives, something Katara did not appreciate when she visited. Arranged marriages are the norm here, with girls becoming betrothed at age 16, and Princess Yue was already engaged when Sokka met her.
What is more, the Northern Water Tribe hobbles its own military somewhat with a "no girls allowed" policy, forbidding any girls or women to practice combat-oriented Waterbending for any reason. Instead, Northern girls are taught how to perform water-based healing, with entire schools set up for this purpose. While it is true that water healing is an essential art, this custom is a waste of many girls' potential as Waterbenders. The tribe's military would be greatly strengthened with well-trained women such as Katara and Hama in its ranks.
By Korra's time, though, the Northern Princess Eska is seen using combat-based bending just like her twin brother Desna. Clearly, the North has softened its stance on women's place in society since Aang's day, and many would agree this progress was overdue. Now the Water Tribe can realize its full potential in an Avatar era where the future means everything.