Wrap Up

Review of the main topics and ideas you encountered in Unit 6.

Water, Rivers, Floods, and Caves: Canyonlands, Delta and Mammoth Cave

• Most rain evaporates especially from plants, most of remainder soaks in.
• Soil and shallow rock usually have air as well as water in spaces; deeper, below water table, spaces all water-filled.
• Water table looks like a smoothed version of the ground surface, and hits surface at streams.
• “Sponge” of ground fills during rains, drains to keep streams running between rains.
• Water table rises during wet times, sinks during dry times.

Rivers Move Rocks

• Rivers get water, and rocks by mass movement.
• If more rocks arrive than water can move, rocks pile up, steepening stream so it can move more rocks.
• Sticky, small clay particles favor a single, deep meandering channel that moves suspended load up in channel.
• 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.
• Which 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, compacts a lot.
• The natural sinking is balanced by new mud from the annual flood.
• Humans hate mud on carpet, so raise natural levees to keep river out of houses.
• But sinking continues—most of New Orleans has subsided below river and 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.
• Scientists, disaster planners, many journalists and others repeatedly warned that this would happen…
• 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 rare so dissolving focused, get sinkholes from surface, caves beneath, springs, etc.=karst.
• If cave dries, loss of soil CO² to cave deposits dissolved limestone as 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 is very hard and slow.
• Some clean-up options, but best to keep poisons out of ground.