 When rivers run into the ocean or into lakes, there is a very distinct change in the flow characteristics and thus the deposition and transport of sediment. And in particular, rivers are narrow channels with a concentrated flow, but the oceans and lakes are much broader standing water. And so the interface between rivers and standing bodies of water are very important sites of deposition. And so depending on the details of that interface, they can be deltas at one end, where there is so much sediment that the rivers build out into the lake or ocean, or estuaries, where the lake or ocean floods into the river valleys. So there are a number of different things that categorize these. So for deltas, the key process is that the river deposits sediment in the ocean or lake. And the river waters running out past the average shoreline. For estuaries, that the sediment that's being transported by the river is captured in the river valley further upstream. So basically in estuaries you have deposits in the river valley itself. So if we look at, let's say we have land up here going all the way across, and our river is flowing in, and we'll make this the ocean. For a delta, there's so much sediment that the river builds out through time, right? So we say that this is a process of building out or progradation. And the term delta comes from the triangular shape that the rivers build out through time. In contrast, a river in an estuary comes to the shoreline, but it either does not contain very much sediment, or that sediment is transported away from the shoreline very quickly, and the lake or ocean water floods up into the valley itself. And so in this particular case, with an estuary, the shoreline goes inward and up that particular valley. So there are three processes that affect whether you have a delta or an estuary, one of which is the sediment supply from the river. The second is the rate of sediment transport away from the shoreline, for example by storms or tides, and then the third is sea level or lake level changes. If sea level starts going up very quickly, even a river with a moderate sediment supply can get flooded because the lake level is rising, and if this valley doesn't get filled entirely with sediment, it'll end up getting flooded. If sea level or lake level goes down, there's less area that needs to be filled with sediment to keep the river valley from being flooded. Similarly with deltas, if sea or lake level goes down, the sediment will be deposited more and more at the edge of the delta. If sea level goes up, this river may have enough sediment to still maintain its outward building shape, but the depositional environments will change in response to that sea level change. Sediment supply can also change based on climate or tectonic uplift, and then the sediment transport within the lake or the ocean changes with the strength of storms and tides through time. Thanks for watching.