 Estuaries are river valleys that are flooded with sea water. They usually form when the sea level is going up and that the higher sea level pushes the sea water back up into an area that used to be influenced and dominated by the river. And so if we think about it in terms of the flow speed, we have the flow of a river confined to the channel and it's very fast and when it reaches the sea water, the area here, the flow speed slows down a great deal and you get deposition of the sediment in this area here. So right at the river mouth edge is where you have a lot of the deposition. So there are several things that can happen depending on the rate of sea level rise. If it's going up, the sediment that's deposited here can just get buried and the estuary will keep filling up the river. At some point there'll be a balance between how much sediment's coming in from the river and how much deposition there is and that will define the boundaries of the estuary. So we still have the main deposition here. The area for the estuary, the furthest upriver point here, is the furthest that you get the sea water intruding and then going down towards the ocean, there are a couple of different ways it can be defined. If there's a straight coastline, the edge of the estuary would be the extension of that coastline or if the coast is quite variable it could be sort of the end of the freshwater. So the oceans have tides in most areas and so there's very commonly when the tide's coming in a flux of sea water into the estuary and the river flow can vary through time as well, usually that seasonally but then you often get fresh water on average that's flowing to the ocean. So if we make a cross section through this area, so if you look at this cross section, we have fresh water coming in from the river and fresh water is less dense than sea water because it doesn't have the salt in it that the ocean has. That lens of fresh water will often float on top of the sea water and the balance between where the fresh and the salty water are within the estuary here will depend on the strength of the tide and the strength of the river flow. These two bodies of water often mix and that produces what we call brackish water which is an intermediate salinity between the fresh water and the salty water. Organisms that live in estuaries have to be able to withstand those big changes in salinity that are associated with changes in this boundary and the mixing and so a lot of times estuaries are ecologically very interesting places with organisms that have adapted to this really dramatic change in water chemistry. So we can look at the sub-environments within estuaries and a lot of times they'll be an area that is more open to flow. So this might have the stronger flow coming out. It usually doesn't end quite that abruptly and then a lot of times if you have a nice warm climate and you can get a lot of marsh and the marsh will vary being more fresh water closer to the river and the source of fresh water and being more of a salt water marsh farther to the south. This would be an estuary that has very little waves or tides in the ocean. If there's a significant amount of tidal flow that's transporting sediment within the estuary the tides flowing in and out through this area where we're getting deposition creates bars and islands that have a geometry a lot like those entirely influenced deltas. So for example, you can get these islands that are elongate parallel to the tidal flow direction. So the ocean processes can also have waves and waves create beaches. So almost always what happens if you have an estuary in a wave dominated zone is that you create a beach here and there's almost always long shore drift because the waves don't come in exactly with the shoreline. So I'm going to say our long shore drift is down in this image. And so what happens is you end up building a set of barrier islands and they can have multiple openings or maybe only one depending on the details of the size of the waves versus the flow of the river and then you create a lagoon behind this area. And again you have deposition in the mouth of the river so sometimes you might even see a little bit of a delta geometry right where the river is flowing into the lagoon. The waves usually don't propagate very far inland in an estuary because it's a flooded river valley. So the depth of the water is really quite low and most of the wave energy from the ocean waves is dissipated along more closer to the average shoreline. And so usually the estuary and the estuary are proper there. There's not a big influence of the ocean waves although wind can produce small waves within those environments. Thanks for watching.