Overview of the main topics you will encounter in Module 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, primarily from plants; most of the water that is not used by plants 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 reaches the surface at lakes and rivers.
The ground acts something like a sponge, with spaces filling during rains, and draining to keep rivers running between rains.
So, the water table rises in elevation during wet times and sinks during dry times.
Rivers Move Rocks
Most of the water that reaches rivers flows through the ground first.
Mass movement supplies most of the rocks that reach rivers.
If more rocks arrive at a river than the water can move away, the rocks pile up, steepening the river flowing away from the pile so it can move more of the rocks.
When the rocks are mostly small clay particles, which tend to stick together, the river often has a single, deep meandering channel, and many small rock particles are transported while suspended up in the water.
When the rocks are mostly larger pieces such as sand, gravel, or even larger chunks that don’t stick together, the river often has many channels that split and rejoin, and is called a braided river; more of the rock transport occurs as bed load bouncing or rolling along the bottom.
Dams Make a Big Difference
When a river carries rocks into a reservoir (the lake behind a dam), the rocks are dropped as a sediment deposit called a delta (deltas also often form where rivers carry sediment into other lakes or the ocean, although strong currents in a lake or the ocean may carry away the sediment, too).
A delta builds out into the reservoir but also builds upward so it continues to slope downward into the reservoir, and this “backs up” sediment that can bury fields and houses for some distance upriver.
Dams typically reduce or eliminate floods farther downriver.
This makes a huge difference for what lives on a floodplain (in particular, it favors humans over nature).
Without floods, big rocks are no longer moved by rivers.
Clean water released by dams picks up small rocks such as sand, removing sand bars and affecting river ecosystems.
Ignoring rivers can be dangerous
A delta is a big pile of sediment, which slowly compacts under its own weight.
The Mississippi Delta is miles thick and compacts a lot.
Naturally the sinking was balanced by new mud from floods, which usually happened every spring.
Humans hate mud in our houses and floods on our roads, so we have built levees, often on top of natural levees, to keep the rivers out of cities and towns.
But the delta continues to compact and sink—much of New Orleans has subsided below river level, and some is beneath sea level.
Wetlands south of New Orleans toward the Gulf of Mexico have been lost as levees and dredging for shipping and oil production have kept floods from depositing mud to balance sinking.
The city now is low in elevation, beside a higher river and sea as human-caused warming raises sea level, with reduced wetlands that would have protected the city if they still existed, and all of this can lead to huge disasters when hurricanes hit.
Before recent hurricane disasters happened, scientists, disaster planners, many journalists, and others repeatedly warned that the disasters were coming.
The sinking of the city and rising of the sea are continuing; rebuilding without major changes will cause the next disaster to be even worse.
Caves are Cool
Some rocks (esp. limestone) dissolve easily; if such rocks start with just a few cracks, the dissolution will be focused in just a few places.
Such focused dissolution makes low spots called sinkholes at the surface, with caves beneath, commonly with springs and other features, giving a landscape called karst.
Caves tend to form near or below the water table and to be filled with water initially, but the water may then drain out leaving the cave air-filled; water dripping in then may 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 farther downriver tomorrow.
Water usually moves through other types of rocks much more slowly so pollution released today may not “get” anyone for a while, but once it does, clean-up is usually 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.