Overview of the main topics you will encounter in Unit 6.
It was kind of solemn, drifting down the big, still river, laying on our backs, looking up at stars, and we didn't even feel like talking aloud..."
— Mark Twain, Adventures of Huckleberry Finn, Chapter 12.
Water, Rivers, Floods, and Caves: Canyonlands, Delta and Mammoth Cave
Most of the rain that falls then evaporates, especially from plants; most of the remainder soaks into the ground.
Soil and shallow rock usually have air as well as water in spaces; deeper, below the water table, the spaces are all water-filled.
The water table looks like a smoothed version of the ground surface, and hits the surface at streams.
The ground acts something like a sponge, with spaces filling during rains, and draining to keep streams running between rains.
So, the water table rises during wet times, and sinks during dry times.
Rivers Move Rocks
Rivers get water mostly soaking through the ground, and rocks by mass movement.
If more rocks arrive than the water can move away, the rocks pile up, steepening the river so it can move more rocks.
Sticky, small clay particles favor single, deep meandering channels that move suspended load up in the channels.
Lots of sand, gravel, or larger rocks that don’t stick together favor many shallow braided channels.
Dams Make a Big Difference
Sediment builds deltas to fill reservoirs formed by damming rivers.
A delta builds out but also up, “backing up” sediment to bury fields and houses for some distance upstream.
Regions downstream of dams no longer get floods after a dam is built.
This makes a huge difference for what lives on floodplain (favors humans over nature).
Without floods, big rocks are no longer moved by rivers.
Clean water released by dams picks up sand, removing sand bars and affecting river ecosystems.
Ignoring rivers can be dangerous
A delta is a big pile of sediment, which compacts under its own weight.
The Mississippi delta is miles thick, and compacts a lot.
The natural sinking is balanced by new mud from the annual flood.
Humans hate mud on their carpets, so raise natural levees to keep the river out of houses.
But sinking continues—most of New Orleans had subsided below river level, and some is beneath sea level.
Wetlands below New Orleans have been lost as levees and dredging for shipping kept flood mud from balancing sinking.
A low city by a high river and sea with no wetlands to slow storm surges brought huge hurricane disaster.
Before this happened, scientists, disaster planners, many journalists, and others repeatedly warned that it was coming…
The sinking continues; rebuilding without major changes will cause the next disaster to be even worse.
Caves are Cool
Some rocks (esp. limestone) dissolve easily; if cracks are rare so the dissolving is focused, then sinkholes form going down from the surface, with caves beneath, springs, etc., giving a landscape called karst.
If the cave then becomes air-filled, water dripping in will lose extra CO2 picked up from the soil, depositing dissolved limestone to make cave formations.
Water goes through caves quickly; pollution discharged today may harm someone tomorrow.
For other rocks, water moves much more slowly; pollution may not “get” anyone for a while, but once it does, clean-up usually is very hard and very slow.
Scientists and engineers have developed some clean-up options, but the best option is to keep poisons out of the ground.