Introduction to Hydrology:Floodplains & Drainage Basins

Lessons 21 & 22

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.

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 1st order stream is the smallest stream in a watershed or

system of streams They do not have tributaries flowing into them

Where two 1st order streams meet, a 2nd order stream is formed

Where two 2nd order streams meet, a 3rd order stream is created

However; when a 1st order and 2nd order stream meet, a 3rd order stream isn’t formed Two 2nd order streams are required to form a 3rd order stream, etc, etc.

Stream Orders, cont.

Stream Orders, cont.

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

http://www.wx4sno.com/portfolio/BSU/spring_2012/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 email 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 27th!