1. Hydrologic cycle`
I. Hydrologic Cycle: the movement of water through and over the Earth
Water continuously traveling between oceans, atmosphere, continents, and mantle (by subduction and volcanism)

2. Hydrologic Cycle
I. Hydrologic Cycle: the movement of water through and over the Earth
97.2% oceans
2.15% glaciers
0.62% groundwater
0.009% freshwater lakes
0.008% saline lakes & inland seas
0.005% soil moisture
0.001% atmosphere
0.0001% rivers
First we’ll examine streams and then groundwater

3. Hydrologic cycle – fresh water inventory

4. 97.2% oceans
2.15% glaciers
0.62% groundwater
0.009% freshwater lakes
0.008% saline lakes and inland seas
0.005% soil moisture
0.001% atmosphere
0.0001% rivers
Hydrologic cycle

5. Question of the week
What is the difference between magnitude and intensity?

6. Streams
II. Stream Terminology
A. Stream (interchangeable with river): any surface water whose flow is confined to a channel.
B. Floodplain: the flat land immediately surrounding a stream channel, which would be submerged if the stream were to overflow it’s banks (wider in valleys than mountains)

7. Streams
II. Stream Terminology
C. Headwaters (Head): the origin or the source water of a particular stream
D. Mouth: the point downstream where the rivers empties into another body of water
E. Tributaries: smaller streams which supply water to a larger stream
F. Trunk Stream: a large stream into which smaller streams carry water and sediment

8. Streams
II. Stream Terminology
G. Distributaries: one of a network of small streams carrying water and sediment from a trunk stream into an ocean (located at the mouth of a stream)
H. Watershed or Drainage Basin: the total area which feeds a particular stream
Every stream, no matter how small, has a watershed (an area that it drains)
Square mile area drained by a small tributary
Mississippi River, 1.25 million square miles basin

9. Or a low ridge dividing the watersheds of two tributaries
Streams
II. Stream Terminology
I. Drainage Divide: area of higher topography which separates one watershed from an adjacent watershed
For example, Rocky Mountains divide watershed of Mississippi from that of the Colorado River
Or a low ridge dividing the watersheds of two tributaries
The most important drainage divide in North America is the Continental Divide. The divide separates streams that flow to the Atlantic from those that flow to the Pacific.

10. Streams
II. Stream Terminology
I. Drainage Divide: area of higher topography which separates one watershed from an adjacent watershed

11. Streams
III. Stream Flow
A. Velocity: the distance the water travels in a given amount of time (varies greatly).
Affects a streams ability to erode and transport material.
Straight channel: fastest in the center, just below the water’s surface.
Curved channel: fastest at the outside of the curve, just below the water’s surface.

12. Streams
III. Stream Flow
Velocity
Depends on:
1. Gradient: slope of the stream, or it change in elevation over a given horizontal distance
Changes within a given stream, usually higher gradients near the headwaters, and gentler gradients at the mouth (general concave-up profile).
All else being equal:
steeper gradients = faster stream
66 m/km to 0.1 m/km
Gradients are measured as vertical drop per horizontal distance)

13. Streams
III. Stream Flow
Velocity
Depends on:
2. Size and Shape of the stream channel
Slowest at the bottom and sides due to friction
Slightly slowed at the surface due to friction with the atmosphere

14. III. Stream Flow
Velocity
2. Size and Shape (continued)
Wetted perimeter: in cross section, the measure of the amount of surface contact between the water in a stream and it’s channel
Faster: small wetted perimeter : cross-sectional area (narrow & deep)
Slower: large wetted perimeter : cross-sectional area (wide & shallow)
When a given stream channel narrows, the stream must move faster
Analogy: put your thumb over a hose
Streams

15. Streams
III. Stream Flow
Velocity
3. Roughness of channel
Slower over rough bed (large boulders).
Water bounces around more, energy put into upward, side-ways and up-stream directions, typically headwaters
Faster over smooth bed (clay, sand), typically further down

16. Streams
III. Stream Flow
Velocity
3. Roughness of channel

17. III. Stream Flow
Velocity
B. Discharge
The volume of water passing a given point on the stream bank per unit of time
Measured as cubic meters per second)
1. Formula for hypothetical rectangular stream channel:
Discharge = width x depth x velocity

18. III. Stream Flow
Velocity
Depends on:
B. Discharge
2. Discharge also depends on:
a. size of drainage basin
b. amount of precipitation
Varies on daily cycles and seasonal cycles
3. In a given stream, discharge increases from head to mouth
4. Amazon: >12 times higher than Mississippi
One day discharge is equal to New York cities water needs for >5 years!
Streams

19. Discharge = width x depth x velocity

20. Discharge = width x depth x velocity

22. Streams

23. Due Monday, March 6th at beginning of class
Problem Set 3
Due Monday, March 6th at beginning of class
Go to and enter 101 in left navbar search field.
On G101 web page, download Problem Set #3, PDF file.
Discharge, stream velocity, wetted perimeter
Simple calculations
Need to understand units of calculations (e.g., m/sec or m3/sec)

24. Streams
IV. Stream Erosion
Most important geologic agent of surface change: they can erode, carry, and deposit sediment
Processes of Erosion: faster stream = more erosion
1. Abrasion: scouring of the stream bed by transported particles
Fine particles suspended in the water constantly scrape the bed
Large pebbles my swirl in local eddies, carving out circular depressions called potholes

25. Streams
IV. Stream Erosion
Processes of Erosion
2. Hydraulic lifting (erosion by water pressure)
Fast, turbulent water dislodges and lifts particles from the channel into the stream
Water forced into cracks with enough pressure to pry up rocks
3. Dissolution
Water may dissolve underlying rock
For example: limestone or evaporites

26. Streams
IV. Stream Erosion
B. Base Level: the lowest level to which a stream can erode its channel
The level at which the mouth of a stream enters the ocean, lake, another stream
Ultimate base level = sea level
Local or Temporary base level = lakes, main streams, dams,
Or resistant layer of rock = waterfalls created, base level is lower on other side of rock
Headwaters: are way above base level actively eroding, downcutting
Near mouth: close to base level with less downward erosion, more side to side erosion

27. Streams
IV. Stream Erosion
C. Valley Geometry
1. Narrow: usually more toward head of stream = high gradients)
Where streams are high above base level = more down-cutting
Streams cut straight down into the underlying rock form valleys with vertical walls
Usually V-shaped due to overland flow and mass movements removing loosened material on the slopes

28. IV. Stream Erosion
C. Valley Geometry
2. Wide: usually toward the mouth of the river = lower gradients
Once a stream has cut near to base level: energy directed to side to side erosion
Widening of valley creates flood plain, either erosional or depositional

29. Streams
IV. Stream Erosion
C. Valley Geometry
3. Channel patterns:
a. Meandering Stream
Current fastest on outside of river bend - actively erodes cut bank
Current slowest on inside of bend - deposits load and creates a point bar deposit

30. IV. Stream Erosion
Current fastest on outside of river bend - actively erodes cut bank
Current slowest on inside of bend - deposits load and creates a point bar deposit

31. Streams
IV. Stream Erosion
C. Valley Geometry
3. Channel patterns:
a. Meandering Stream
Meanders progressively get more pronounced
Finally only separated by a thin strip
Stream may form a cutoff
Abandoned meander becomes an oxbow lake

32. Streams
IV. Stream Erosion
C. Valley Geometry
3. Channel patterns:
b. Braided Stream
Networks of converging and diverging stream channels
Separated by narrow sand and gravel bars
Develop in steams with very high sediment loads
More sediment load than its carrying capacity

33. Streams
IV. Stream Erosion
C. Valley Geometry
3. Channel patterns:
b. Braided Stream
Deposit their load in the stream bed  build up until they’re islands
Occurs: Where stream banks are loose unconsolidated material (lack vegetation) or
Where slope of stream decreases suddenly, base of mountain, steep tributary flowing into a gentle gradient main stream
End of glacier = abundance of glacial sediments