The Work of Streams The Work of Streams

The Work of Streams

The Work of Streams • Streams generally erode their channels lifting loose particles by abrasion, grinding, and by dissolving soluble material. http: //soer. justice. tas. gov. au/2003/image/108/pwater_erosion-stream-m. jpg

The Work of Streams • Streams transport materials of erosion in three ways: 1. In Solution (dissolved load) 2. In Suspension (suspended load) 3. Scooting or Rolling along the bottom (bed load)

Dissolved Load • Most of the dissolved load finds its way into streams via groundwater. • Usually expressed as dissolved material per million parts of water (parts per million).

The Work of Streams • Streams deliver almost 4 billion metric tons of dissolved substances to the oceans each year. Photo by R. Jowsey

Suspended Load • Most streams carry the largest part of their load in suspension. • The visible cloud of sediment suspended in the water is the most obvious portion of a stream’s load. http: //rpmedia. ask. com/ts? u=/wikipedi a/commons/thumb/1/10/Skawa_River %2 C_Poland%2 C_flood_2001. jpg/215 px Skawa_River%2 C_Poland%2 C_flood_20 01. jpg

Suspended Load • Streams usually only carry sand, silt, and clay in this fashion. • Higher velocity in a stream may allow it to carry larger particles in suspension. http: //www. geomorph. org/gal/mslattery/IAG 4. jpg

Bed Load • Bed load is that part of a stream’s load that is too large to be carried in suspension. http: //www. medinaswcd. org/images/streamsediment. jpg

Competence and Capacity • The ability of a stream to carry a load is determined by two factors: 1. Competence 2. Capacity

Competence • Competence of a stream measures the largest particles it can carry. • A streams competence increases with velocity. http: //amyglenn. com/images/stream_load. gif http: //uregina. ca/~sauchyn/geog 323/hjulstrom. gif

Capacity • A stream’s capacity is the maximum load it can carry. • Capacity is directly related to a stream’s discharge. • The greater the volume of a stream, the greater its capacity for carrying sediment.

Deposition • As a stream’s velocity decreases (slows down), its competence decreases and sediment begins to drop out, largest particles first. Click for Deposition Animation

Deposition http: //www. northcoastjournal. com/011603/cover 0116 sediment. jpg • Deposition occurs as streamflow drops below the critical settling velocity of a certain particle size.

Sorting • Stream transport separates solid particles of various sizes, large to small. • This process is called sorting. • It explains why particles of similar size are deposited together.

Alluvium http: //www. windows. ucar. edu/earth/geology/images/all uvial_fan_lg. jpeg • The sorted material deposited by a stream is called alluvium. • Many different depositional features are made of alluvium.

Deltas • A delta is an accumulation of sediment formed where a stream enters a lake or ocean. http: //upload. wikimedia. org/wikipedia/commons/th umb/e/ef/Indus_River_Delta. jpg/300 px. Indus_River_Delta. jpg

Natural Levees • A natural levee is a ridge made up mostly coarse sediments that parallels some streams. http: //sepmstrata. org/MARINESEDIMENTS/delta/Image 5. gi f

Natural Levees • A natural levee forms when a stream repeatedly overflows its banks. http: //www. google. com/imgres? imgurl=http: //www. uwsp. edu/ge. O/faculty/ozsvath/images/natural_levees. jpg&im grefurl=http: //qwickstep. com/search/naturallevees. html&usg=__2 Hl. Ikw. Orvs. T_z 58 p_1 xf. L 1 D 9 BBc=&h=357&w=1020&sz=66&hl=en&start=0&zoom=1&tbnid=Vb Og. Xh. XPx 0 mz. M: &tbnh=48&tbnw=137&ei=bboz. Tc 6 YM 42 cg. Qe. Gl. K 2 HCw&prev=/images%3 Fq%3 Dnatural%2 Blevee%26 hl%3 De n%26 safe%3 Doff%26 client%3 Dfirefox-a%26 rls%3 Dorg. mozilla: en. US: official%26 gbv%3 D 2%26 biw%3 D 1227%26 bih%3 D 518%26 tbs%3 Disch: 10%2 C 126&itbs=1&iact=hc&vpx=483&vpy =250&dur=398&hovh=133&hovw=380&tx=184&ty=55&oei=bboz. Tc 6 YM 42 cg. Qe. Gl. K 2 HCw&esq=1&page=1&ndsp=21 &ved=1 t: 429, r: 17, s: 0&biw=1227&bih=518

Narrow Valleys • A narrow v-shaped valley indicates that a stream’s primary work has been down -cutting towards base level.

Narrow Valleys • The Yellowstone River is an excellent example of a narrow valley. • Rapids and waterfalls are the predominant features of a narrow valley. Photo by R. Jowsey

Narrow Valleys Photo by R. Jowsey

Narrow Valleys Photo by R. Jowsey

Wide Valleys • Once a stream has cut its channel closer to base level, downward erosion decreases. • More of the stream’s energy is then directed from side to side. • The result is a widening of the valley as the river cuts away at first one bank and then the other.

Floodplain • The side to side cutting of a stream eventually produces a flat valley floor, or floodplain. http: //www. uwec. edu/jolhm/EH/Disrude/images/floodpla in. gif

Floodplain • Streams that flow on floodplains move in meanders. • A meander is a looplike bend in the course of a stream. http: //1. bp. blogspot. com/_m 4 za 0 XNgt. DQ/ TPe. PP 8 VFdn. I/AAAAANc/ozhg. Ntd. WX 0/s 400/okvango+river+meander++robhig. jpg

Meanders • Once a meander begins to form, it gets larger. • Most erosion occurs on the outside of the meander. http: //www. scioly. org/w/images/6/65/Meander. jpg

Meanders • The outside bank in a meander is sometimes called the cut bank. http: //www. scioly. org/w/images/6/65/Meander. jpg

Check for Understanding • Why is the cut bank (outer bank) the area of greatest erosion in a meander? The cut bank is the zone of greatest stream velocity.

Oxbow Lake • An oxbow lake is a crescent-shaped (often temporary) lake that is formed when a bend in a river is cut off from the main channel by the forces of erosion. http: //www. geosociety. org/Earthcache/Images/For ming%20 an%20 Oxbow%20111232009. jpg http: //www. nywetlands. com/oxbowlake. html

Oxbow Lake • An oxbow lake will slowly be created as soil erodes and re-deposits, changing the river's original course. http: //homepage. ntlworld. com/harry. wickens/borneo/bo rneo-070. jpg

Oxbow Lake • Oxbow lake formation - video demonstration (click left). http: //belmont. sd 62. bc. ca/teacher/geology 12/photos/erosionwater/meander-oxbow. gif

Independent Assignment • Use your notes and your knowledge of biology to answer the questions on your worksheet.

References • Adapted from: Tarbuck & Lutgens (2009). Earth Science. Prentice Hall, Boston. http: //www. nywetlands. com/oxbowlake. html