The vast swamp forests that covered the Illinois Basin during the Pennsylvanian were complex environments. Many developed on large river deltas that fanned out from highlands to the north. Streams in the deltas flowed in multiple channels that divided and came back together like the strands in a braid. These streams were bordered by high levees built by sediment-rich floodwaters. Swamps and lakes filled the spaces between. There were also brackish (salty) estuaries (inlets) bordering the shallow seas.