Dynamic Planet/Earth's Fresh Water


 * This page is for the 2011–2012 Dynamic Planet topic. For the 2009–2010 topic, see Earthquakes and Volcanoes.

Earth's Fresh Water is the topic for Dynamic Planet for the 2011 and 2012 seasons. It is centered on rivers, lakes, and groundwater.

The Basics
Freshwater is water that has less than 0.2% of dissolved salts. Comparatively, freshwater is relatively scarce, comprising less than 3% of the Earth’s water supply. In addition, the majority of that, about 2% is frozen away in glaciers and icecaps. Fresh water can also be found in lakes, rivers, streams, atmospheric vapor, and groundwater. Only about 0.5% of the Earth’s water is available for human and animal use, as atmospheric vapor, salt water, and icecaps are unavailable for use. The availability of freshwater is also limited by population, competition, and pollution. Further Information on Freshwater

Erosion
Water causes 4 basic types of erosion:
 * Rain splash results from the impact of water droplets, forming small indentations
 * Sheet erosion occurs when water runs off in sheets, stripping away a layer of material
 * Rills and gullies form when water runs off in trickles, eroding a small channel deeper and wider (rills are just smaller forms of gullies)
 * Stream erosion occurs at the sides of streams by hydraulic action

Hydrologic Cycle
There is a common steady cycle of water throughout the Earth. The best way to understand this is to follow the image below.



More Info
The hydrologic cycle, better known as the water cycle, describes the movement of water through the hydrosphere. The easiest place to begin is with evaporation. As the sun heats up the Earth's surface, water evaporates, meaning it changes from a liquid to a gas, and enters the air. Another important way that water vapor can enter the atmosphere is through transpiration, which is the loss of water from parts of plants, mainly their leaves. Once water vapor is in the atmosphere it goes through the process of condensation, where it returns to a liquid state and forms clouds. When the water droplets in the clouds become large enough they will begin to fall to the ground as precipitation. Then, when the precipitation reaches the ground some of the water will become run-off and flow to a river or other body of water. Also, some water will infiltrate the ground and become groundwater, where it can replenish aquifers. Eventually the water will again evaporate and the cycle will continue.

Evaporation
More Info

Water Budgets
Basically, sum the total input and subtract the total output to get the water budget for a watershed. Very good lecture on water budgets

Also, have a general idea about the amount of water in each of its forms on earth: in lakes, rivers, oceans, clouds, ice, etc. These are pretty good figures.



Streams
Streams are a general name for all moving water.

Stream Drainage
Streams follow a general pattern based on topography. Drainage Channels form where runoff cuts into the ground.

Profile
In the case for large rivers, a delta or mouth of the river at sea level is a "Base Level", in fact, sea level itself is considered the "Ultimate Base Level". How can a waterfall be a base level? These pictures should shed some light on it.



This picture shows a longitudinal profile, or a general profile of a river as compared to distance and elevation. As you can see in this picture, The origin is at the highest elevation, while the mouth is at the ultimate base level. By looking at this graphic, we can make some general assumptions:

1. The closer to the origin you are, the faster the water will flow

2. The closer to the mouth you are, the slower the water will flow

3. Sediment will be scoured closer to the higher elevation

4. Sediment will be deposited at the lower elevations

5. There is a higher stream gradient the closer to the origin you go

6. There is a lower stream gradient the closer to the mouth you go

So how does this work into waterfalls? Let me show you with another concept: Downcutting!

Downcutting is the deepening of a river channel relative to its surroundings. That is, how far does it dig into the ground. As natural examples tell us, The amount of downcutting on a river is dependent on where on the river it forms. Look at this example:



This picture shows what downcutting looks like on a normal river. At point “A”, the river is very fast moving and at a higher elevation to that of sea level, so it downcuts at a steady rate. At point “B”, the river is slowing down some, and is getting closer to sea level, so downcutting is considerably slower here than at Point “A”, and at Point “C”, downcutting is almost non-existent. However, science has shown us that downcutting does not continue down to sea level at the same speed in all cases. This is where we dive into the base level features.

Let’s review what we have determined so far:

Base level is the closest to sea level a river can go. Downcutting helps a river in its descent to ultimate base level.

Now, if downcutting doesn’t always continue to sea level, what blocks its path? Well, in order to understand this, we have to add a little onto our definition of a base level. Base level is the closest to sea level a river can flow at any one location. In other words, in real time, the base level at Point “A” on our graphic could be different to the Base Level of point “B”. It all depends on the rock layers. This is where we get into the final focus point.

At any one time, rock layers can dictate base levels.

Geologically speaking, nothing impedes downcutting. However, at our timescale, we can witness downcutting happening before our very eyes. That is essentially what a waterfall is, an agent of downcutting. Look at this graphic of a waterfall:



By looking at the graphic, we can determine a definition. A waterfall is a morphological feature defined by water flowing over a hard rock layer. In the case of most waterfalls, the water that flows over the falls erodes the softer layer at the base. Once it erodes enough, the unsupported hard layer above collapses. This is what makes a waterfall appear to “retreat”. So how does this fit into river morphology? It acts like a new point of origin. Look at this final graphic:



In actuality, this is what a longitudinal profile looks like, if you were to make it precise. Though mine is a sloppy mess, hopefully you can see what I’m trying to get across. It has these stair steps, base levels, that act as a mini origin, restarting the morphological process. These don’t have to be waterfalls. Lakes, other rivers, and even man-made dams have this kind of effect.

Alluvial Landforms
Once a stream reaches a base level it forms a large fertile valley due to its meandering:



Flood Plain
A flood plain is the flat area that tends to be covered in water when the river rises. As a flood increases the rivers size it slows the river down causing it to drop sediment which in turn allows for very fertile soil.

Natural Levee
A natural levee is formed when sediment(alluvium) is deposited along the edge of the stream forming a ridge

Meanders
A meander is a bend in a stream. Meanders are prevalent in older streams. Meanders have erosion on the outer bank and deposition on the inner bank.



A point bar forms where the water going through a meanders drops alluvium on the inner bank

The neck is the point of land between the two edges of a meander.

The cutoff occurs when the stream erodes through the neck causing the river to be back to a straight course.

The result is an Ox-bow lake which is a separate body of water from the stream



Deltas
Deltas are formed when rivers meet large bodies of water like oceans. They are classified as follows:


 * Constructional
 * river-dominated
 * elongate (digitate, bird foot delta)
 * lobate
 * Destructional
 * tide-dominated
 * wave-dominated
 * cuspate (tooth shaped delta)

Other names for types of deltas: Arcuate delta, Estuarine delta



Stream Order
Strahler developed a system known as stream order that helps determine how many tributaries a particular stream has and how large that stream is. Basically, start with the smallest stream, which has no tributaries; this is a first-order stream. Two first order streams join to make a second order stream, and so on. If a second-order stream joins a first-order stream, the resulting stream is still second-order.

This image explains it in a picture rather than in words:



Lakes
Lakes are stratified into several layers based on temperature.

Lakes are also classified by their nutrient levels and oxygen concentrations.

There are many ways of forming a lake

Groundwater
Groundwater is water that is in the ground. It exists in the pore spaces and fractures in rock and sediment. It originally was rainwater or snow. Water will move down into the earth until it reaches a layer of soil where it can not penetrate. This layer is called the impenetrable or impermeable layer. The uppermost reaches of this water is called the water table.

Facts: Further information on Groundwater
 * Groundwater makes up about 1% of the water on Earth. That's about 35 times the amount of water in lakes and streams.
 * It occurs everywhere beneath the Earth's surface, but is usually restricted to depths less that about 750 meters.
 * The surface below which all rocks are saturated with groundwater is the water table.

Hydraulic Head
The depth of groundwater in two different places, when measured, can give hydraulic gradient, basically a calculation of slope.


 * hydraulic gradient (I) = (h1 - h2) / d

where h1 and h2 are two different heights, and d is the distance between them.

This allows us to calculate the velocity of groundwater flow, if we know porosity (a unitless percentage that describes what percentage of a rock, gravel, or sand is empty) and permeability (the variable K).


 * groundwater velocity (V) = hydraulic conductivity (K) x hydraulic gradient (I) / porosity

Darcy also found out how to determine groundwater discharge using this information and the area the water flows through.


 * discharge (Q) = hydraulic conductivity (K) x hydraulic gradient (I) x area (A)

Some resources on groundwater discharge and aquifers

More resources on groundwater discharge and aquifers

Links

 * 1) Online Book
 * 2) All the topographic map symbols: usually a pretty big topic on the tests
 * 3) Extra information on streams and soil water
 * 4) Best resource on lake composition
 * 5) Good alternative to wikipedia
 * 6) Particular information on pollution
 * 7) A short overview that's a good introduction to streams; all the must-know information that you shouldn't have to put on the resource
 * 8) More notes, similar to the Online Book above but covering groundwater in more detail