Chapter 7. Drainage Basins

Describe the issues related to defining a "first-order" channel.

"First-order" channel is a channel that regularly carries flow. Stream ordering is usually done from maps and is not based on fieldwork; the scale at which the basin is considered is critical.

Explain the factors leading to each of the four common drainage patterns.

1) Dendritic pattern - absence of structural or lithologic controls. 2) Trellis and rectangular patterns - govern my geological structures and the strength contrast between different rock types. 3) Radial patterns - influenced by topography.

Draw a series of sketches to illustrate the difference between superposed and antecedent drainages.

Antecedent drainages - rivers predate the deformation that accompanied development of a mountain range. Drainage was able to keep pace with uplift and maintain the river's prior cource as the mountains rose around it. Superposed drainages - river course developed on sedimentary cover erodes into and is imposed on the underlying bedrock as the cover gradually erodes. How the F*$K am I suppose to sketch this?!

Discuss the residence time of sediment in a drainage basin.

I have no idea how to answer this question and its not in the book.

Describe the difference between open and closed drainage basins.

In OPEN drainage basins, mass (water, sediment, and the dissolved load) is transported through and then out of the watershed either to larger watersheds or to the ocean. CLOSED drainage basins (also known as internally drained) terminate in lowlands, where water is lost only by evaporation and seepage into the groundwater system.

Why and how does discharge change down-stream in an arid-region river network?

In arid basins the relationship between drainage and downbasin is more complex because losing streams are common, and thus channels need not convey increasing discharges downstream.

Predict how channel width, depth, and cross-sectional area change downstream in a humid-temperate river network.

In humid-temperate regions, discharge increases downstream with increasing basin area and so do channel width, depth, and cross-sectional area.

Compare and contrast the Shreve and the Strahler stream ordering classifications.

In the Strahler classification of stream ordering, stream order increases only when two streams of the same order come together. When two first-order channels join, the channel downstream is designated as a second-order channel. The addition of lower-order streams does not influence the order of the main stream. In the Shreve classification of stream ordering, stream magnitude increases every time a tributary stream enters the main stream. This approach defines stream magnitude as the total number of first order streams contributing to the reach in question. Look at Fig. 7.5 on page 226. It will make more sense.

Define both a knickpoint and a knickzone; explain how they are different and what can cause them to occur.

Knickpoints are discrete jumps in elevation along a river's bed, or waterfalls. Such jumps commonly retreat and grow less steep over time. Knickpoints can result from base level change, faulting, resistant rocks, or the lingering effects of valley glaciation. Knickzones are areas where the bed of the river is steeper than up or downstream - a cascade or area of fast water. Such oversteepened reaches can reflect faulting or the presence of strong rocks that are resistant to erosion.

Define a sediment rating curve and describe how sediment rating curves are created.

Sediment rating curve describes sediment discharge as a function of water discharge. Sediment rating curves are created by estimating flow over time (water discharge, x-axis, m^3/sec) and measuring suspended load during a variety of different discharge evens (sediment discharge, y-axis, kg/sec).

Where in drainage basins does sediment tend to be deposited?

Sediment tends to be deposited on relatively flat lowlands, oceans and deltas. as colluvium at the base of hillslopes (50%) in valley-bottom terraces (30%) yielded at river mouth (20%)

Where in drainage basins does sediment most likely originate?

Sediment tends to be originated in upland (hillslope) erosion (70%), from tributaries (20%), and from landslides (10%).

Where is stream velocity greater - in a mountain cascade or a large lowland river? Explain your answer.

Stream velocity is greater in a large lowland river because deep wide lowland rivers flowing through relatively smooth channels have little friction along the bed and banks, allowing water to move quickly despite the low slope.

Drainage networks

The form of the drainage network is controlled in part by the earth materials underlying the basin, in part by the tectonic setting of the watershed, and in part by the history of the channel network itself. Geology, topography, or tectonics.

Sketch four common types of drainage patterns and suggest a location where each type might be found.

1) Dendritic pattern - consist of a system of branching tributaries that form a treelike pattern fanning out toward the basin headwaters. Tend to form in the absence of structural or lithologic controls; occur in relatively flat-lying sediments or in homogeneous crystalline rocks. 2) Trellis drainage pattern is characterized by two dominant channel orientations in which primary tributaries join main channels at roughly right angles, with secondary tributaries running parallel to main channels. Form in areas underlain by tilted or folded beds of alternately weak and resistant sedimentary rocks, where preferential erosion along weak beds results in development of bedding-parallel strike valleys with short, steep, orthogonally oriented dip and antidip streams incised into the more resistant strata. 3) Rectangular drainage pattern is similar to, but more symmetric than trellis drainage, with the two dominant drainage directions more equally developed. Characterizes areas in which jointing or faults govern drainage patterns by producing linear zones that are more susceptible to weathering and erosion; such weaknesses are particularly common in landscapes underlain by carbonate rocks, such as limestones. 4) Radial drainage patterns consist of channel networks that flow away from or toward a central point. Radial drainage networks typically reflect flow off symmetrical landforms such as volcanoes or structural domes.

Describe three specific challenges geomorphologists face in creating a sediment budget.

1) Temporal variability: many slope erosion processes, such as landslides and gullying, are episodic, and short-term and long-term mass flux rates often differ dramatically. 2) Estimating the volume of sediment stored in depositional landforms (floodplains, terraces, fans, and colluvial deposits). 3) Estimating the ages of above mentioned landforms and the sediment they contain. OR according to Larry: 1) Estimating the amount of upland (hillslope) erosion. 2) Estimating how much sediment has been stored in depositional landforms (see examples above), for how long and how much sediment have been removed from depositional landforms. 3) How much sediment has been yielded out of the lowlands.

Define a drainage basin

A drainage basin is the entire area drained by a stream and its tributaries. Drainage basins (watersheds) are the land surfaces over which water and sediment move down topographic gradients.

Describe how both average basin slope and channel slope change as a function of basin area.

Basin area is inversely related to both channel and basin average slope.

Explain why data in sediment rating curves are so variable.

Because sediment discharge vary greatly over time and from event to event.

In what climate and tectonic setting are you most likely to find closed drainage basins?

Closed drainage basins are most commonly found in arid regions where drainage networks are not well developed, in areas where the local tectonic regime is extensional, and in glaciated areas where glacial erosion has overdeepened valley bottoms. Famous examples of closed drainage basins: Great Salt Lake (for you Mike) and Dead Sea.

Explain why fine-grained and coarse-grained sediments have different average residence time in humid-temperate drainage basins.

Coarse grained sediments are harder to move, hence residence time for those sediments will be higher than fine-grained sediments.

List the four valley segment types and describe the dominant processes in each.

Colluvial valleys: found in the headwaters of drainage networks, where hillslope processes deliver sediment to valley bottoms by soil creep and rare episodic processes like debris flows and landslides. Bedrock valleys: narrow, steep valley bottoms with little sediment storage and a high capacity for sediment transport. Alluvial valleys: low gradient and filled with sediment, streams are usually unable to scour to bedrock. Floodplains and terraces are common and can store significant amounts of sediment. Estuarine valleys: the interface between the terrrestrial and marine realm, these low-gradient valleys are filled with fine-grained sediment, are wide, and are heavily vegetated.