The number of individual organisms that die in a population in a given year
the movement of individuals into a population

That's called a birthrate. The number of individual organisms born into a population in a given year.

Immigration is the movement of individuals into a population

Explanation:

(while emigration is the movement of individuals outside of a population)

answer: tributaries flow into rivers.

wetland is a biome that develops when the water gets accumulated in a land exhibiting a huge depression. these form either seasonally or permanently. the water in the wetland comes from rivers, lakes, ocean, flood, rain or other sources. the wetlands includes mangroves, marshes and peatland.

tributaries flow into the rivers develops a wetland because such water will get deposited in the depression and will restrict the original flow of tributary water. hence, water with low drainage current combines with the river water.