Review the Unit 6 Introduction
You have reached the end of Unit 6! Double-check the list of requirements on the Unit 6 Introduction page and the Course Calendar to make sure you have completed all of the activities listed there.
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.
Reminder - Continue to work on Exercise #3. See Course Calendar for specific due dates.
Following are some supplementary materials for Unit 6. While you are not required to review these, you may find them interesting and possibly even helpful in preparing for the quiz!
- Website: Canyonlands National Park
- Website: Delta National Wildlife Refuge
- Website: Mammoth Cave National Park
Comments or Questions?
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