The Pacific Flyway terminates in the northern tundra regions of central Canada and Alaska. Sand from the ocean, and rock, minerals and organic material from fresh water creeks swirl constantly into the lagoon. Because sediments are heavier than water, they tend to settle out of the water, a process called sedimentation. Where these sediments finally settle effects the nature of the lagoon, its ecosystem, and its future. When the sediment deposits build to a height that severs the sea's influence, the lagoon dies.
Many factors affect sedimentation.
For example, because fresh water is lighter than seawater, creek sediments don't mix with the denser, saltier water of the lagoon but 'float' towards its center. The faster the creek flows, the further the sediments will be carried away from the creek mouth. During times of heavy rains creek sediments may even be carried directly out to the ocean, with no chance to settle within lagoon boundaries. Slower creek flows encourage settlement within the lagoon. When creek flow is low, sediments settle near creek mouths, often at the far reaches of the lagoon.