GY 111 Lecture Notes D Haywick 2008 09 1 GY 111 Lecture Note Series Sedimentary Environments 2 Rivers and Deltas Lecture Goals A Rivers and Deltas types B Water Flow and sedimentation in river deltas C Sedimentary rock types Reference Press et al 2004 Chapter 7 Grotzinger et al 2007 Chapter 18 p 27 431 436 442 GY 111 Lab manual Chapter 3 A Rivers and Deltas types Before we get too far it is probably best to state right away that rivers and deltas are separate depositional systems and should be discussed individually We are combining them here because of time constraints That being said there are legitimate reasons to consider them together in a GY 111 class as rivers and deltas are related Rivers are channels that contain water that is flowing down hill The flowing water is carrying sediment and that sediment is eventually transported to large standing bodies of water like lakes and where we live the oceans When the flowing water of the river channel meets the sea it immediately slows and this results in a tremendous amount of sediment deposition The deposit that forms can be 100 s of km2 in extent and is called a delta see picture to left of the Mississippi Delta Both rivers and deltas contain channels and in both river and delta systems the channels come in different sizes and shapes Rivers are considered to be the largest of the channels Creeks streams and ditches are all smaller versions of rivers For now let s focus in on rivers In Alabama almost all rivers streams and channels contain water all the time We live in a wet climate and the water that flows in channels largely comes from rainfall In drier climates e g the SW USA streams periodically dry up These ephemeral streams are common elements of arid regions and deserts around the word Water may not flow through these streams all the time but when it does flow say after an infrequent major thunderstorm it flows really fast Many a camper has been swept away through these sudden surges If the water flows over the banks of the river it exceeds the carrying capacity of the stream we say that the river has flooded If the water rise and flooding event is quick sometimes only a matter of minutes it is called a flash flood By the way I should mention that while it is true that most streams in Alabama receive the majority of their water through rain fall this is officially called runoff some of the water gets into channels through ground water This process is especially common in arid regions and is one of the reasons why rivers like the Colorado contain water even after flowing hundreds of miles across deserts Some of the water leaks into the channels through groundwater infiltration GY 111 Lecture Notes D Haywick 2008 09 2 There are many types of rivers but the two most common are 1 meandering as pictured to the left and 2 braided pictured to the right The two form in response to different slopes Meandering rivers form in areas of shallow slope braided rivers form on steeper slopes The term slope really is meaningless when applied to rivers It is better to use the term gradient which is defined as vertical drop feet horizontal distance mile The gradient is reported as a number and the units are retained A steep gradient might be 300 to 1200 ft mile whereas a shallow gradient might only be 50 or 60 ft mile Rivers are capable of flowing down very shallow gradients only a few feet per mile but the way they flow is not consistent On steep gradients the water flows in an almost straight line Sometimes the channel is straight but more often than not it forms a braided pattern with numerous sand gravel bars breaking up the flow Thus is born a classic braided stream If the gradient is very shallow the stream forms a classic meandering pattern Channels in deltas are almost always meandering because the gradient at sea level where deltas form is so low however like rivers deltas do come in different flavors We recognize 3 distinct varieties based upon major morphological characteristics and the dominant depositional process at the site of deposition 1 Bird s Foot e g Mississippi Delta form where waves and tides are weak 2 Wave dominated e g Nile Delta form where waves are strong 3 Tide dominated e g Ganges River Delta form where tides are extreme GY 111 Lecture Notes D Haywick 2008 09 3 We are talking about depositional environments so you are correct in suspecting that sediment is deposited in both river and delta environments Sediment is actually deposited in two different components of rivers deltas 1 the channel and 2 the flood plain We ll tackle each of these in the next section of this lecture but I have to warn you that some of the explanation requires an understanding of water flow The study of water flow is one of the subject areas of hydrodynamics which can at times require differential equations and statistics Luckily for use this is not one of those times B Water flow in meandering channels The cartoon below is a cross section of a meandering channel It could be across a river channel or a delta channel The process of water flow is the same regardless of the depositional environment in question Note that in the cartoon the channel is not symmetrical The banks of the channel have a steep side the cutbank side and a shallow side the point bar This asymmetry generates a prominent corkscrew motion to the water flowing in the channels In other words water spirals across the channel as the water flows down stream Why is this important The corkscrewing motion circulates water from the deep part of the channel just adjacent to the cutbank to the shallow part of the channel adjacent to the point bar The deepest part of the channel is where the water flows the fastest and the shallowest part of the channel is where it flows the slowest So the corkscrewing motion has the effect of moving water from the fast part of the current to the slow part Now as we have discussed previously flowing water is capable of transporting sediment The faster the water flow the coarser the sediment that is transported It stands to reason therefore that if the water in a meandering channel corkscrews from fast flow to slow flow the coarsest sediment it erodes when moving fast deep channel is quickly dropped deposited when the current slows in the shallowest part of the channel To make a long story short sediment is eroded from the cutbank side of the channel and sand is deposited on the point bar side of the channel This is in fact the reason that