GEO 101: PHYSICAL GEOGRAPHY Chapter 14: River Systems and Landforms

Transcription

1 GEO 101: PHYSICAL GEOGRAPHY Chapter 14: River Systems and Landforms Topics Fluvial Processes and Landscapes Streamflow Characteristics Floods and River Management

2 Fluvial Processes Fluvial = stream-related processes fluvius ( Latin ) = river

3 Rivers are the Earth s lifeblood Most civilization occurs along major rivers and migrates with the change of rivers

4 Functions of Rivers: The artery for distributing fresh water Redistributing mineral nutrients (plant growth) Transporting weathering products (shape landscapes) Deposition of sediments (fertile soils for farming) Cooling Power generation Transportation networks Political boundaries

5 Major Rivers in the US? Longest Rivers in the world?

6 Dynamic Equilibrium View of Landforms A landscape is an open system: With highly variable inputs of energy and material potential energy (gravity), heat energy (solar), kinetic energy (mechanic motion), chemical energy (reaction between water, atmospheric gases and crust), etc.

7 Dynamic Equilibrium View of Landforms Endogenic events (e.g., Earthquakes, volcanic eruptions, etc.) and Exogenic events (heavy rainfall or forest fire) may lead to the system to reach a Geomorphic Threshold, break through the system to a new equilibrium.

8 Dynamic Equilibrium View of Landforms The changing pattern through time would follow: (1) equilibrium stability (fluctuating around some average) (2) a destabilizing event (3) a period of adjustment (4) development of a new and different condition of equilibrium stability This is the basic principle for monitoring and modeling the change of landscapes

9 Earth Systems Science: Concepts and Theory System Equilibrium: An energy and material system that remains balanced overtime. Dynamic equilibrium Metastable equilibrium Dynamic Metastable equilibrium Threshold: A critical point at which the system can no longer maintain its character and lurches to a new operational level

10 Fluvial Processes and Landscapes Fluvial Erosion: weathered materials are dissolved or removed and transported to new locations Deposition: settling down materials carried by running water

11 Two types of flows: Laminar flow Turbulent flow Fluvial Transport Stream loads: Suspended load Solution load Bed load Bed loads transport: Rolling Traction (drag) Saltation (jump) Abrasion among particles causes the decrease of grain sizes and increase of roundness downstream Figure 14.13

12 Fluvial Processes and Landscapes Base Level of Streams: A level below which a stream cannot erode its valley (John Wesley Powell, 1875)

13 Fluvial Processes and Landscapes The degradation of a landscape by weathering, erosion, and transportation will ultimately reduce the landscape down to the local base level. Figure 14.3

14 Drainage Basins Drainage basins are divided by ridges that form drainage divides Drainage divides have different scales, from continental scale to kilometer scale Drainage divides define watershed water-receiving drainage basins Figure 14.4

15 Fluvial Processes and Landscapes Drainage Density Drainage Density: The total length of all stream channels in the basin divided by the area of the basin Humid climate: High drainage density Desert : Low drainage density

16 Fluvial Processes and Landscapes Drainage Patterns Drainage Patterns: The arrangement of channels in an area. Seven common drainage patterns Drainage patterns are controlled by geologic and climatic conditions Slope Differing rock resistance Climatic and hydrologic variability

17 Gentle slope and relatively uniform rock types Drainage Patterns Folded mountain belt valley and ridge provinces One river erodes through a drainage divide and captures the headwater regions of another stream Domes such as a volcanic Mountain Steep slopes Faulted and jointed landscape Structural domes Glaciated regions Figure 14.9

18 Streamflow Characteristics Stream Discharge: The volume of water per unit time Q = wdv Q=discharge w=stream width d=stream depth v=velocity

19 Effects of Stream Discharge Q = wdv Question: if we keep building the levees to prevent floods along the river, what would happen? (assume the discharge is constant) Figure 14.10

20 Stream Gradient If the gradient of a stream increases, the stream will begin to erode its channel and/or its banks Given enough time, erosion would stop at the base level. Figure 14.18

21 Streamflow Characteristics Exotic Streams : discharge decreases with distance from source due to high evaporation rates in the arid area downstream e.g., Yellow river, Nile River Natural processes along streams Weathering erosion transportation deposition

22 Streamflow Characteristics Three types of Stream Channels: Straight Braided Meandering

23 Braided Stream When sediment load is heavy relative to its discharge sediment deposition sand and gravel bars forming numerous small channels Figure 14.14

24 Meandering Stream Development Figure 14.15

25 Figure 14.23

26 Meandering Stream Profile Erosion of river banks in one side and deposition at the other side point bar development Figure 14.15

27 Itkillik River, Alaska Figure 14.15

28 Mississippi River Delta Problems of the Mississippi river delta: Huge sediment supply cause subsidence Oil pumping and pipelines cause subsidence as well Major shift in the river's course now exists New Orleans could become a "city without a river." Figure 14.28

29 Floods and River Management Rating Floodplain Risk: Historical data, statistics and measurements Streamflow Measurement 11,000 stream gaging stations (USGS) Figure 14.30

30 Urban Flooding Hydrograph Urban pavement increases runoff reduce the lag time between storm peak and discharge peak behavior like deserts!! Harder to predict and take actions Figure 14.31

31 End of Chapter 14 Fluvial? Functions of Rivers? Major Rivers in the US? Longest Rivers in the world? Endogenic events and Exogenic events Dynamic Equilibrium View of Landforms: the basic principle for monitoring and modeling the change of landscapes Fluvial erosion and deposition, Types of flows and fluvial transport, Base level of streams, local and ultimate base levels Drainage basins, density, patterns, drainage divides, watershed, factors controlled drainage patterns Stream discharge and stream gradients Exotic streams; Natural processes along streams; Types of Stream Channels Floods and River Management; Hydrograph; Urban Flooding