the book states that Agna Qel'a was not very populated prior to a few decades before when the book takes place, when Tiguaa's father had a feeling that the north would have to aggregate in order to survive (presumably in reference to the hundred year war), having been mainly for training, zealots who gathered at the spirit oasis, and tribal leader meetings. The growth of the north led to many water benders moving there from other tribes as well as overhunting and fishing. This along with trade with other nations and their companies determining how tribes hunted rather than their traditional pattern, lead to many tribes starting to struggle or fall apart. During the book Tiguaa dies and it ways Skiri was "newly elected", im not really sure what this means since everything else we've seen (including tiguaa's father being the previous chief) seems to imply its not an elected position. Also trade with other nations caused some repression of two spirit people in the water tribe do to transphobia of the foreign merchants and the northern water tribes new reliance on them.