Rock On #6
What happens to most of the water that falls on central Pennsylvania's Happy Valley each year (or any similar place, such as Washington, DC or other places with trees)?
It is re-evaporated, mostly after passing through trees. Feedback: Water gives life, and life is very good at using water. When their leaves are out, trees use almost all the rain that falls, and tree roots reach down into the ground and pull up some of the water from cold-season rain and snowmelt. An important amount of water does soak into the ground and flow to streams, but plants still get the majority. Flow across the surface is increasing as we pave the landscape, but most of the land is not paved, and flow across natural surfaces to streams is small. Streams are tiny, so direct rainfall into them is small. And soft-drink plants use only a tiny bit of water compared to total rainfall.
In the map above, blue shows the Mississippi River, and the Gulf of Mexico, around the Birdfoot Delta of the river. The USGS image uses different colors to indicate changes in the delta. Orange and red both indicate change in one direction, whereas yellow and green indicate change in the other direction. Based on the material presented in this class:
Orange and red indicate loss of wetlands over time, whereas yellow and green indicate gain of wetlands over time. Feedback: The Mississippi Delta is sinking below the waves, so the widespread orange and red must indicate loss of wetlands.
What happened in the picture above?
Rivers have delivered sediment to the sea, forming deltas that built up as they built out so that they still slope slightly downhill toward the sea. Feedback: Two deltas have formed where streams carry sediment from the hillside into the fjord in South Greenland. The main source of sediment is the streams from the land, not suspended sediment already in the sea water. And the deposits cannot be purely flat-topped, or the rivers would not flow across to get to the sea water in the fjord. Mollusks do live along the coast but are not being protected by intrusive changes to the coastline, nor do enough people visit here to be worth advertising Delta Airlines.
Large rivers have many interesting features, including:
The flood plain, the nearly flat region farther from the river than the natural levees and composed of mud deposited by the river's floods. Feedback: Many processes contribute to the formation of flood plains, but deposition of mud to smooth the surface is the most important one. Flood plains often occur beyond natural levees. The initial slowdown as floodwater spreads from a river channel into the trees deposits sediment to form natural levees.
Near Aaronsburg, PA, a company wanted to start a limestone quarry, and planned to pump lots of water out of the ground to make things fairly dry near the quarry so it wouldn't fill with water. Concern was raised—would this affect the nearby trout streams? So, a little harmless dye was placed in a sinkhole next to the proposed quarry, and a fire-engine pumper added a lot of water to the sinkhole. How long did it take, or will take, for the dye to reach the trout stream?
A few hours to days Feedback: The dye showed up in a few hours, and the quarry was not excavated. Sinkholes often connect directly and quickly to underground caves or big cracks, and thus to streams, allowing rapid drainage. There are rock units that would hold their water for centuries or millennia, but such units have small spaces, not caves and sinkholes. Local sinkholes do drain to trout streams, and Michigan has to make their own water pollution because water pollution from Pennsylvania does not reach them. (Fun thing to do if you're bored: fit this question into the Michigan fight song.)
If you went swimming in the single channel of this river, and grabbed a sample of the river bank, what would you likely come up with?
Clay, that sticks together and can hold up steep slopes Feedback: This is a meandering channel, and these normally are fairly deep and narrow, so the materials of the banks should be able to stick together and support a steep slope. Sand can be steep when damp, but slumps to nearly flat when wet, and boulders or too much sand plug streams and make a braided pattern, whereas clay can make very steep slopes.
When humans build or raise levees along big rivers such as the lower part of the Mississippi, we are likely to cause:
Fields and roads on the flood plain to drop below the surface of the river, because compaction of flood-plain mud will no longer be balanced by sediment accumulation during floods. Feedback: "Long, long, time ago, I can still remember how the river used to mend with mud..." The continuing compaction of the sediment along the lower Mississippi should be balanced by new deposits on top; stopping the new deposits does not stop the compaction, so the surface of the flood plain sinks. Even racing around curves, rivers don't tilt much, and mud compaction doesn't do much to meandering. "With a pink carnation and a pickup truck..."
Given the materials presented in this class about the formation of caves, it is likely that most large caves are formed:
In limestone in moist climates Caves require easily dissolved rock, and water to dissolve that rock. In really dry climates, limestone is a resistant rock that stands in huge cliffs. In wet climates, limestone dissolves to yield caves. Sandstone is not a good cave-former because sandstone does not dissolve easily. (Yes, there are very shallow rock-shelter caves in sandstone, which is why the question specifically notes "large caves".) And while Diet Pepsi actually would be marvelous at dissolving limestone, Diet Pepsi attacks granite rather slowly and won't make caves well.
Both of the above pictures are along the Colorado River. The clear water of picture 1 and the muddy water of picture 2 appear quite different.What's going on?
2 is upstream of the Glen Canyon Dam, and 1 is downstream of the dam. Feedback: The naturally muddy river is seen clearly in Canyonlands in 2. The river dumps its sedimentary load in the reservoir above the dam, so downstream of the dam the water is clear, as shown in 1.
The Mississippi River:
Has built a delta, which is several miles thick at its thickest point, from near St. Louis, MO to the Gulf of Mexico over millions of years. Feedback: Amazing as it may seem, the Mississippi has been taking the debris from the vast area from the Rockies to the Appalachians, and dumping that debris into the Gulf of Mexico, building a pile of sediment that is miles thick in places and extends from St. Louis to the Gulf. The mud has filled an old crack in the continent from when the Atlantic and Gulf of Mexico opened, but the mud doesn't stop the earthquakes that occasionally occur near the tip of the crack. And as for the Yoo Hoo, Yuck!
Years may pass with no major damage to the US mainland from hurricanes, but other years bring huge damages. A terrible event happened in 2005, when levees around New Orleans failed in the rising waters of Hurricane Katrina. More than 1400 people died, and the damages were in the neighborhood of $300 for each person in the US, or about $100 billion. As discussed in the text, history shows that:
Scientists and serious planners had warned about such an event for decades, based on the known size of hurricanes and the sinking of the Mississippi Delta and much of New Orleans. Feedback: Scientists and planners did not know exactly when a big hurricane would hit New Orleans and threaten the city, but serious assessments had consistently highlighted the possibility for decades. The failures at New Orleans happened despite the fact that Katrina was NOT the "Big One"—on a scale of 1-5, Katrina was a 3 when it made landfall.
A dam is built on a river, forming a reservoir. Over time, this likely will cause:
Sedimentation to bury farmer's fields upstream of the reservoir, and erosion of sand downstream of the dam. Feedback: The stream will slow where it enters the new lake, and so will deposit sediment to form a delta rather than cutting downward or having no change. As the delta builds out into the lake, the upstream end of the delta must build up so that the stream still slopes downward, and this will tend to bury fields upstream. Meanwhile, moving water can carry sediment. Sediment-free water is released from a dam but often later observed to have sediment, so erosion must be occurring. Loss of sand bars below the Glen Canyon Dam shows that sand is carried away downstream of dams. Dams stops floods that are needed to move the big pieces (boulders, cobbles), and dams cause sedimentation upstream, but not downstream. Our friends from Columbus are probably too cultured to be relieving themselves outside the Mall; besides, the last time I looked during a visit from the Buckeyes, the inebriated people were not straying that far away from the downtown bars.
Geologically speaking, the water table:
Separates the water-filled region below the Earth's surface from the region closer to the surface in which some air exists in the spaces. Feedback: The water table is the surface below which all the spaces are full of water, but above which there is generally some air in the spaces. During droughts, water drains away from the ground to the creeks, so air enters spaces previously occupied by water, and the water table drops in elevation. Creeks do change in elevation between rain and drought (floods happen...). And while there might be bottled water in the Capitol, geologically speaking, that is not the right answer.
In the picture above, Dr. Alley is discussing events that are happening outside of Grand Canyon National Park, which may impact the park. What are the issues he is discussing?
Water pumped out of the ground for golf courses and other uses evaporates, so less water flows through the ground to the springs of the canyon Feedback: Water soaks into the ground on the plateaus beside the canyon, seeps down to hit a rock layer that blocks the flow, and flows along that layer to feed beautiful and biologically important springs in the Canyon. Pumping water out of the ground on the plateaus to use for humans generally allows the water to evaporate (say, from the grass of a golf course) or run down a stream (say, below a sewage treatment plant), so the water doesn't flow through the ground to the springs.
Suppose that all the rainfall that fell during an average year on a typical surface in central Pennsylvania just stayed there as a layer of water (and all the snow melted, and the melt just stayed there). If at the end of the year you were standing on your head on that surface (assuming you are a typical-sized human being), what would be true? (In an average year, Pennsylvania gets about the same amount of precipitation as the average for the world.)
You would be breathing by SCUBA or snorkel, because the water would be up between your belly button and your knees somewhere. Feedback: A typical rainfall supplies about an inch of water, or just under 0.1 foot. 30 feet of rain would be a big storm every day, about equal to the wettest place on Earth, and while sometimes it may seem the rain in Pennsylvania will never end, there really are clear days. 0.3 feet is a mere 3 or 4 rainfalls per year, and is a dry desert. 0.03 feet would be the driest place on Earth, and 0.003 doesn't occur on Earth. 3 feet is a nice number, which would be up to your knees or so if you were standing on your head, so you'd need the SCUBA gear.
