1) Arms 2) Firths 3) Rias 1) Coastal waters 2) River mouths 3) River-sea junctions 4) Where seas and rivers meet 
• (pl. ) of Estuary
(Learning Modules / Mathematics / Bridges) The mouth of a river where it empties into the sea. Due to erosion, estuaries are often extremely wide. This width, coupled with the tidal currents up and down the river, require great expertise to span with a bridge.
areas near the ocean where salt and fresh water mix together