The features of an estuary are determined by a region's geology, and influenced by physical, chemical, and climatic conditions.For example, movements in the Earths crust elevate or lower the coastline, changing the amount of seawater that enters an estuary from the ocean.he amounts of seawater and fresh water flowing into an estuary are never constant. The quantity of seawater in an estuary changes with the changing tides, and the quantity of fresh water flowing into an estuary increases and decreases with rainfall and snowmelt.