I am found in the mouth of the river when sediment is dropped by the river . What am I?

I'm thinking that it's a delta.

Explanation:

Deltas are formed when sediment that has been deposited at the mouths of rivers. Although, I might be wrong idk.

significant amounts of nitrogen oxides, carbon monoxide, and other pollution

explanation:

passenger vehicles are a major pollution contributor, producing significant amounts of nitrogen oxides, carbon monoxide, and other pollution. in 2013, transportation contributed more than half of the carbon monoxide and nitrogen oxides, and almost a quarter of the hydrocarbons emitted into our air.

x would be the most and the least would be y because there are more fruit   and seeds than flock z has insects