Frank Press • Raymond Siever • John Grotzinger • Thomas H. Jordan Understanding Earth Fourth Edition Chapter 14: Streams: Transport to the Oceans Lecture Slides prepared by Bill Dupré • Peter Copeland 1 2 Copyright © 2004 by W. H. Freeman & Company Types of Sediment Transport Fine-grained sediment carried in suspension • Suspended Load – Fine-grained sediment (typically clay and silt) transported in suspension due to turbulence • Bed (or traction) Load – Coarser-grained sediment (typically sand and gravel) transported on the bottom of the stream bed by rolling and sliding • Saltation – Sediment (typically sand) transported by intermittent jumps - a transitional state between bedload and suspended load. coarse-grained sediment slides and rolls as bedload. 3 Figure 414.2 Saltation (intermittent bouncing) Increased velocity increases suspended load and… a transitional state between bedload and suspension increases bottom shear stress, increased bedload. Figure 514.2 Figure 614.2 Waterfall formed Potholes by form by headward erosion pebbles and gravel grinding inside eddies Figure 714.5 Figure 814.6 The Two Main Types of Channel Meandering River in Alaska Patterns on Floodplains are: • Meandering Streams – have a single channel with a sinuous pattern – are the most common pattern on floodplains • Braided Streams – have an interlacing network of channels – are relatively uncommon 9 Point Bar Meandering channel Figure10 14.9 Meandering Streams Lateral migration of meandering streams… Erosion on the cutbank Deposition on the point bar Figure11 14.9 Figure12 14.9 …eventually results in meander cutoffs Meandering Rivers Gradually Change Their Course by Lateral Migration and the formation of oxbow lakes. Meander neck Point Bars Meander Cutoff Oxbow Lake Figure13 14.9 Figure14 14.9 Meandering Rivers Abruptly Change Their Course Meanders in the Mississippi River delta by Meander Cutoffs During Major Flood Events Meander cutoff Oxbow Lake Figure15 14.9 Figure16 14.9 Braided River in Alaska Variables that Encourage Channel Braiding Include: • highly variable water discharge • large sediment load • easily eroded bank material Braided channels Figure17 14.9b 18 Low Discharge Period High Discharge Period (e.g. summer) (e.g. spring snowmelt) Figure19 14.9b Figure20 14.9b River valleys are built by Building a Floodplain, One Flood at a Time two processes Low Natural Levee • Lateral accretion: by the lateral migration of bar deposits (mainly sands and gravels). • Vertical accretion: by the deposition of natural levee and flood basin deposits on the floodplain during periods of overbank (flood) flow (mainly silts and clays). 21 Figure22 14.10 Building a Floodplain, One Flood at a Time Building a Floodplain, One Flood at a Time Overbank flow results in the flooding of the floodplain Decreased flow velocity results in deposition of suspended sediment Figure23 14.10 Figure24 14.10 Stream Cutting Caused by Uplift Results in the Formation of River Terraces Figure25 14.17 Figure26 14.17 Terrace Terrace Uplift can also result in the entrenchment of meandering streams, forming “incised meanders” Figure27 14.17 Figure28 14.8 Drainage Divide marks the edge of two adjacent drainage basins Drainage Basin • An area of land that funnels all water that fall on it into a network of streams • The boundaries of the drainage area are called divides 29 Figure30 14.18 Typical Erosional Drainage Patterns Dendritic Drainage Pattern Figure31 14.20 e.g. flat-laying or homogeneous rocks Figure32 14.20 Rectangular Drainage Pattern Trellis Drainage Pattern e.g. jointed or faulted rocks Figure33 14.20 e.g. folded layered rocks Figure34 14.20 Radial Drainage Pattern e.g. volcano or dome Figure35 14.20.