Rivers Yukon and Charley Rivers

Topics 1. Introduction to fluvial processes 2. Types of channels 3. Large Meandering Rivers 4. Understanding & Interpreting fluvial forms

1. Introduction to fluvial processes Streams “work” by eroding, carrying and depositing sediment

Solid load: suspended load & bedload

Suspended Load Little Colorado River

Large bedload, carried during monsoon rains in Nepal Constant suspended load of glacial flour (ground up rock)

Classroom Resources Movies showing bedload transport of gravels Movie showing sand transport & ripple formation

Classroom Resources Load_Fluvial.swf – animation showing types of load carried by streams

Classroom Question In Lord of the Rings (popular with students), Isildur died, slipped from his finger into the big river, and the ring stayed put for over a thousand years -- before it was found by Gollum. How is this possible, given that the ring can be seen in bedload?

Answer comes later - when students learn about oxbow lake formation – perhaps where ring found Image of Mississippi River

Also dissolved load

Suspended Load Khartoum Dissolved Load Suspended Load

Manaus Dissolve Load Organic Acids! Manaus Suspended Load

Erode (degrade)? Carry (transport)? Deposit (aggrade)?

Erode? Carry? Deposit? 200 cm/s ~ 4.5 mi/hr

Elements Of Channel Formation Elements of Channel Formation – One key function of streams is to move the products of erosion to lower, downstream locations. Erosion begins in a Source area that can involve eroding bedrock or reactivating earlier transported material. Sediment from the Source is then Transported through the stream system The channel that is formed during this Transportation process will reflect the material balance that exists. When the system has no net loss or gain of sediment (this condition is often referred to as being “In regime”), the channel will usually be relatively stable vertically but can experience considerable lateral shifting. This lateral shifting is most common in meandering streams. Finally, the sediment must be deposited somewhere within the system. This deposition often occurs just upstream of a site where Base Level Control occurs. Normally, braiding and multiple channels form in the depositional area and the stream has unstable channels. Diagram used with the permission of Geomax, P.C., Spokane, WA

Erosion in headwaters Transport in middle Deposition closer to ocean

High gradient (mountain) streams – erode and carry large bedload (high competence)

High competence load creates potholes & other evidence of abrasion

Middle – mixture of erode, transport, deposit, and for much of the year – like this Depends on flood or not

Lower end - deposit Mississippi River delta has very high capacity (lots of sediment, Just small competence)

These are generalizations that work for larger system and not smaller pieces

Organisms influence streams, especially at reach & smaller areas See Online Video

Flow Velocity Relationships Velocity Relationships – Sediment transport is highly sensitive to local flow velocity (turbulence). For example increasing flow velocity from 4.5 feet per sec. to 9 feet per sec. will increase the water’s transport potential from a 1 lb. rock to a 90 lb. rock. Diagram used with the permission of Geomax, P.C., Spokane, WA

Note: always concave upward profile – characteristic of streams Even these small washes

Base Level & Grade Grade – concave upward profile The ability of a stream to erode is based on velocity of water. Velocity is proportional to slope. At a sufficiently low slope, streams will run without eroding: this is called the base level. The ultimate base level is sea level, although local base levels exist, since flow paths are seldom one consistent slope.

Profile – concave upward Base Level Base level is a short-term concept. Change in sea level, for instance, can either increase or decrease slope of a stream profile. Dams create artificial local base levels, and in so doing force steeper slopes (and hence erosion) downstream.

Effects Of Dams From FEMA Effects of Dams -- When a dam is placed on a stream, the sediment transport capability of the stream is disrupted. The dam becomes a local base level control. As the water slows at the head of the dam, any sediment that was being transported tends to be deposited and a delta is formed. The water released immediately downstream from the dam contains considerably less sediment than it did before the dam was built. This released flow is often termed “hungry water” because it usually triggers local erosion of the bed and banks of the channel below the dam. Most dams also tend to reduce peak flow by storing water during high runoff periods. Unregulated Side Drainages that enter the dammed stream also carry sediment loads. Prior to dam construction, these sediment loads normally were flushed down the main stem during peak flows. When the peak flows are reduced by the dam, the sediment coming from the side drainages cannot be flushed away as efficiently. When this happens an alluvial fan tends to form at the mouth of the unregulated side drainage. These alluvial fans then pinch the main stem flow and cause a slackwater area to develop above the fan and rapids to form in the pinched section. An additional problem develops if the dam is used as a peaking power facility. Peaking power facilities require the flow from the dam to be rapidly changed to generate power at critical times (usually in the morning at breakfast and in the evening at supper time). These rapid changes of flow cause excessive bank seepage forces to develop when rapid drawdown occurs. These seepage forces coming out of the bank often destabilize the banks and sloughing can occur. Diagram used from FEMA presentation with the permission of Geomax, P.C., Spokane, WA From FEMA

Classroom Resource Dam Break at Kaui – showing erosion when discharge and velocity increases

When base level drops, every tributary incises because streams are steeper and have more energy

When Grand Canyon formed, it “entrenched” meanders Base level dropped ~ 5 million years ago, freezing in place the old stream patterns

Classroom Resource Spinaround (QTVR) at Goosenecks of the San Juan River

2. Types channels 3 basic types of channel patterns: Straight Channels Meandering channels Braided Channels

Straight – in nature, requires a weakness (fault, jointing) for stream Straight – when you see, usually from human activity (channelized, Nasca, Peru)

Meandering Meandering rivers typically contain one channel that winds its way across the floodplain. As it flows, it deposits sediment on banks that lie on the insides of curves (point bar deposits), and erode the banks on the outside of curves.

Point Bar Cut Bank Point Bar Cut Bank Asymmetrical

Meandering Streams – Streams that are vertically stable normally develop meanderings patterns. Valleys usually are wide and flat. The meander loops shift back and forth across the floodplain as the system transports the sediment load downstream that enters the valley. Meandering always flow in alluvial valleys, tend to armor their beds and create slack water deposits on the floodplain. Diagram used with the permission of Geomax, P.C., Spokane, WA

Braided Channel

Braided Rivers exhibit numerous channels that split off and rejoin each other to give a braided appearance. They typically carry coarse-grained sediment down a steep gradient

Classroom Resource Lab showing movement among braided channels “Spin around” – can see braided stream

Braided – carries mostly bedload, so stream spreads out to bring high velocity close to bed Meandering – carries much suspended load, so stream narrows to carry load more efficiently

Can get very complicated

3. Large Meandering Rivers

Meandering stream, Phnom Penh, Cambodia Oxbow lake Point bar Cut bank

Zooming into a Reach Riffle Pool

Poor Bridge Entry Problems -- When meandering streams “fold in” on roads that cross the floodplain as shown in the previous slide, a number of problems develop. The sharp curve that occurs causes the inside of the bend to become depositional. When this happens, it is common to find a loss of conveyance dropping significantly through the bridge opening. This loss of conveyance can then cause water levels above the bridge/road crossing to become higher during periods of peak flow. Additionally, upstream bank erosion often accelerates and the bridge abutment on the outside of the bend is subjected to increased flow turbulence and can be undercut by excessive bed scour. Diagram used with the permission of Geomax, P.C., Spokane, WA

Bank Erosion at Cut Bank

Bank Erosion Newaukum River (WA)

Classroom Resources

Point Bar Deposit

Point bars

Floodplains

A Floodplain These are satellite images before and during Summer, 1993 floods of the Mississippi river north of St. Louis.

Classroom Resource Animation of before and after 1993 floods

Floods In natural streams, floods occur periodically when seasonal discharges grow extremely large and waters exceed channel banks. The first thing that happens after water goes over bank is the velocity drops, and sediment is deposited. This makes natural levees along channels.

Yazoo Stream

Classroom Resource NaturalLeveesBuilt.swf – animation showing building of natural levees from successful flooding by a meandering stream

Societal Problem Living below the river! Cities grow & obligation to protect Classroom Resource Folder – movie of 1991 Flooding

Certainty: levee will always break. Only question: when

Tendency to increase meander amplitude over time

Classroom Resources Animated maps showing gradual change and also sudden shifts (avulsion)

Tendency towards avulsion (sudden change in channel) Sacramento R.

Oxbow lakes

Classroom Resources

Back to the student hook: perhaps an avulsion occurred soon after the ring slipped through Isildur’s ringer?

4. Understanding & Interpreting fluvial forms (or something to do on a family vacation)

4. Understanding & Interpreting fluvial forms Basic Stream Valley between Interfluves

V-shaped valley, Amazon Headwaters Interfluve (ridge) V-shaped valley, Amazon Headwaters Interfluve (ridge)

Eroding river valleys – v-shape

Knickpoints

Rivers work to destroy knickpoints 1st. Erode down & create v-shaped valleys 2nd. Push knickpoint up stream by undercutting Niagara Falls

Classroom Resources Reconstruction showing Niagara Falls Retreat Flume Movie

Fill Stream Terrace: Stream cuts through its old fill

ASU is on a Fill Terrace

Terrace Breaks Used to Irrigate

Troy Pewe

Why does a stream incise? Uplift of mountains Lowering of sea level (during glaciations) More discharge (extreme flooding events) –creates arroyos in the southwest

Terrace – created by repeated uplift of mountains Terrace – created by repeated uplift of mountains. Strath: stream erodes floodplain on bedrock and then gets additional energy to erode into its old floodplain

Strath Terrace – can see the erosion of the bedrock by the stream (not eroded strata)

Delta: Nile River Wave-driven longshore drift redistributes sediment along the coast.

The Mississippi Delta Jetties have been built and dredging occurs to allow large vessels to navigate the delta channel Note the large amounts of sediment that surrounds the delta A bird’s foot delta located in Louisiana adjacent to the Gulf of Mexico

Deltas Distributaries in deltas periodically shift to find the most efficient path to the sea. Delta-related wetlands are under stress from sediment starvation and lack of natural flooding.

The Atchafalaya Delta This is one of the few areas where the Louisiana coastline is growing This delta is considered to be the Mississippi’s largest distributary The Atchafalaya diverts large quantities of water from the Mississippi

Tide-Dominated Delta Indus Delta Tidal channels and flats Tidal channels widen toward sea

The Irrawaddy River Delta Sediment plumes mangrove forest Fertile mud and sand have been deposited here during the last 2 million years This river delta in Burma is one of the world’s great rice producing regions

Wave- & Tide-Influenced Delta

Online Videos

Online Visualizations Flood Visualizations http://serc.carleton.edu/NAGTWorkshops/visualization/collections/flood.html River Systems: Process and Form http://serc.carleton.edu/NAGTWorkshops/visualization/collections/rivproc.html Waterfall Formation http://serc.carleton.edu/NAGTWorkshops/visualization/collections/waterfalls.html River Erosion, Deposition, and Transportation Visuals http://serc.carleton.edu/NAGTWorkshops/visualization/collections/erosion_deposition.html Deltas http://serc.carleton.edu/NAGTWorkshops/visualization/collections/deltaplumes.html

Imagery seen in this presentation is courtesy of Ron Dorn and other ASU colleagues, students and colleagues in other academic departments, individual illustrations in scholarly journals such as Science and Nature, scholarly societies such as the Association of American Geographers, Geomax presentations for FEMA, city, state governments, other countries government websites and U.S. government agencies such as NASA, NPS, USGS, NRCS, Library of Congress, U.S. Fish and Wildlife Service USAID and NOAA.