Not only did it become possible to farm river deltas without having to resettle frequently as sea level rose, but the infilling of river valleys slowed, making it possible to farm flood plains as well. The Imperial Valley of California is below sea level because it is a former part of the Gulf of California cut off by the Colorado River delta. At one time it was almost entirely dry, but about 1910 an irrigation canal from the Colorado River overflowed and flooded a large part of the center of the valley. By the time the flood was stopped, residents decided that a lake might not be a bad idea, so it is still there. It's called the Salton Sea and is maintained artificially.
Some deltas can actually be inland.
The Okavango River of Africa flows into a closed basin, where it soaks into the ground or evaporates. The Okavango Delta is one of the world's great wildlife habitats. Other inland deltas include the Niger where it first enters the Sahara Desert, and the Sacramento-San Joaquin delta in California. Thousands - Millions of Yr. Damming Tectonic - Lake Kioga Lake Kioga in Uganda is a particularly interesting case of a tectonically dammed lake.