Introduction to Hydrology: Floodplains & Drainage Basins Lessons 21 & 22 Introduction to hydrology  Introduction to hydrology  Stream types, orders, and gradient  Floodplains and landforms

Stream Systems  Streams are bodies of water that are confined to a small region such as within stream banks  Streams produce erosion, known as fluvial erosion  Streams are also important for groundwater recharge and wildlife  A stream may also be called a branch, brook, creek, crick, lick, bayou, wash, run, etc.

Stream Systems, cont.  Usually, a stream begins at a location known as the headwaters  This source location can be a spring or seep or a low area in a valley where water drains to  Recall, water flows from high to low elevation  Small streams flow downhill and converge into larger streams  The point at which two streams merge is called the confluence  The opposite of confluence is bifurcation, where one stream splits into two separate streams

Stream Systems, cont.  As streams converge and diverge, they create a hierarchy of streams with different gradients, lengths, drainage areas, and total volume of water

Types of Streams  Perennial streams are those that have flowing water 90% of the year in a well-defined channel  Intermittent streams are those that have flowing water only during the wet season ( ≤ 50% of the year)  Ephemeral streams generally have flowing water only after heavy rainfall events (e.g. thunderstorms)  Winterbourne streams flow only during the winter months

Stream Characteristics  Streams located on gentle slopes with low water velocities will flow back and forth, finding the lowest elevation to flow to  This shift in the streams course is known as meandering  Over time, a stream will meander back and forth across the flat valley floor, known as a floodplain  Lateral erosion, shifts the course of the stream over time

Stream Characteristics, cont.  This lateral erosion is concentrated along the outside bank of the meandering stream  Primarily due to higher water velocity  However, on the inner bank (slower moving water), soil and silt is being deposited  Known as fluvial deposition  Over time, lateral erosion and fluvial deposition gradually shift the course of the stream back and forth across the floodplain  This results in the newer stream course being steeper, generally in a more straight direction with a faster flow

Stream Characteristics, cont.  Erosion along the outer bank eats away at the soil (1)  Further erosion forms a cut-off meander and a new stream/river channel (2)  Eventually an ox-bow lake is created that is separate from the stream/river (3)

Stream Characteristics, cont.

Fast current = erosion

Stream Characteristics, cont. Slow current = deposition

Stream Characteristics, cont.  Oxbow lakes may eventually become swamps, known as oxbow swamps  If precipitation is low, these swamps may dry up and form small streams or dry stream beds known as meander scars

Floodplain Landforms  Several types of landforms are associated with larger streams and rivers and the floodplains they produce

Floodplain Landforms, cont.  Hills or ridges along each side of a major stream or river are known as bluffs  If river flooding occurs, it generally does not reach this level  Undercut banks or natural levees are formed from silt/soil deposits along the outer edge of the stream/river

Floodplain Landforms, cont.  Streams that flow parallel to the main stream or river are called yazoo streams  Yazoo streams are cutoff from the main stream due to natural levees  When a stream/river meanders and cuts into itself it is called a cutoff meander  This allows for the stream/river to take a shorter course

Floodplain Landforms, cont.  A cutoff meander will initially become an isolated lake known as an oxbow lake  When cutoff meanders fill with sediment and dry up, the become what’s known as meander scars

Floodplain Landforms, cont.  As a stream/river cuts across the narrow neck of a meander (a), the cutoff river bend becomes an oxbow lake (b), which over time becomes an oxbow swamp (c), which in turn becomes a meander scar (d). From McKnight and Hess.

Drainage Basins  A drainage basin or watershed is an area within which all water flows toward a single stream  Watersheds are separated by what’s known as a drainage divide  This is usually a ridge or some sort of man-made divide

Stream Orders  Looking at an entire watershed we can see many streams which have various lengths, sizes, and patterns  These streams can be analyzed through stream ordering  A 1 st order stream is the smallest stream in a watershed or system of streams  They do not have tributaries flowing into them  Where two 1 st order streams meet, a 2 nd order stream is formed  Where two 2 nd order streams meet, a 3 rd order stream is created  However; when a 1 st order and 2 nd order stream meet, a 3 rd order stream isn’t formed  Two 2 nd order streams are required to form a 3 rd order stream, etc, etc.

Stream Orders, cont.

 Notice that the elevation gradient changes much faster between 1 st and 2 nd order streams than it does between 3 rd and 4 th order streams downstream

Quick Review  Homework problems ask that you evaluate several topographic profiles. I highly suggest you refresh your memory and revisit lessons 5 and 6, which can be found online at: /lectures/GEOG101L_Lessons_5-6.pptx

Quick Review, cont.  Furthermore, recall that contour lines in the shape of a “V” indicate a stream or creek…with the apex of the “V” pointing uphill  There are 63,360 inches in one mile  There are 5,280 feet in one mile  Page 139: Problem 2a-2d, bonus worth 1pt.  If you need help at all, please me to setup an appointment or come see me during office hours…

Office Hours  Wed: 11:30 AM to 12:30 PM  Thr: 9:15 AM to 12:30 PM  Fri: 10:00 AM to 12:00 PM  Meet briefly next week to turn in Lessons 21 and 22.  If you have questions about your grades, see me no later than April 27 th !