Review Questions and Answers; Rivers and Streams

The oceans are the main reservoir for water and a good starting point for discussing the hydrologic cycle (Fig. 10.3). Water from the oceans evaporates and eventually falls as precipitation (rain, snow, dew, etc.) on land or into the sea. Precipitation on land can evaporate back into the atmosphere, flow as runoff into streams and rivers, infiltrate into the soil and bedrock to recharge the groundwater, or be frozen into glacial ice. Groundwater discharges as springs or seepage flow to perennial streams, and glacial ice eventually melts. Thus freshwater derived from land areas eventually returns to the ocean, completing the hydrologic cycle. Glacial ice volume has an important regulatory effect on sea level. As ice caps formed and grew larger during the Pleistocene glacial epochs, sea level fell, and as the ice sheets melted and shrank, sea level rose.

2. Over the oceans, evaporation exceeds precipitation. Why does sea level not drop?

Most precipitation originates by evaporation from the oceans. Over time, water evaporated from the oceans is replenished by inflow of freshwater from rivers and streams. Continental ice sheets and glaciers have a strong effect on sea level changes. Expanding glacial ice volumes result in lowered sea level, shrinking ice sheets and glaciers effect rising sea level.

3. List several factors that influence infiltration capacity.

Textural properties of the surface material, kinds and abundance of vegetation, topography, and delivery mode of the moisture all have important effects on infiltration capacity.

Permeable, initially unsaturated, highly porous regolith can hold up to 30 percent or more of its volume as water when fully saturated. Of course, the infiltration capacity of any porous material declines as the percentage of unsaturated pore space declines. Impermeable, surficial materials such as massive bedrock and asphalt paving prevent infiltration so moisture runs off or evaporates. Water infiltrates more readily into moist, unsaturated regolith than into dry regolith.

Dense, vegetative cover enhances infiltration because soils are typically moist and porous, thus runoff is retarded. In forested areas, trees slow down the rate at which precipitation is delivered to the land surface, and considerable moisture is temporarily stored in humus and forest litter. On gentle, tree covered slopes and flat lying terrain, slow runoff enhances infiltration. Runoff is accelerated on steep slopes and in areas with sparse vegetation, and infiltration decreases accordingly.

The rate at which water is delivered to the land surface has a very important effect on infiltration. Short-lived storms with intense rainfall result in lower infiltration and increased runoff because the water "piles up" on the surface faster than it can infiltrate. During periods of light to moderate rainfall, runoff is retarded and infiltration rises accordingly. Special environmental conditions, such as snow melting above frozen ground, greatly intensify runoff and reduce infiltration; conversely, snow melting above unfrozen ground can result in a high percentage of the moisture infiltrating the soil. Thus regional and local climatic factors are also important.

4. "Water in streams moves primarily in laminar flow." Briefly explain whether this statement is true or false.

Essentially, all open channel flow of water is turbulent, not laminar; thus the statement is false. However, most groundwater flow is laminar, except in karst areas where underground streams may be present. Turbulence describes the transitory, swirling water currents in flowing fluids such as water and air. It largely accounts for the erosive power and sediment-moving capacity of streams, and upward currents in turbulent eddies can lift sediment particles off the bottom and keep the smaller ones in suspension.

5. A stream originates at 2000 meters above sea level and travels 250 kilometers to the ocean. What is the average gradient in meters per kilometer?

The gradient is the drop in elevation of the stream divided by the length of the flow path. Thus the gradient is 2000 m/250 km or 8 m/km.

6. Suppose that the stream mentioned in Question 5 developed extensive meanders so that its course was lengthened to 500 kilometers. Calculate this new gradient. How does meandering affect gradient?

The new gradient would be 2000 m/500 km or 4 m/km. If a fairly straight channel should develop meanders, the flow path would lengthen without any change in the elevation drop; thus the gradient is lowered.

7. When the discharge of a stream increases, what happens to the stream's velocity?

The average velocity (V = Q/A; discharge/area of water cross section) also increases.

8. What typically happens to channel width, channel depth, velocity, and discharge from the point where a stream begins to the point where it ends? Briefly explain why these changes take place.

In stream and river systems with orderly, increasing discharges in the downstream direction, the channel parameters (width and depth) and average velocity increase gradually to accommodate the increased discharge moving through the channel. Rivers that flow from wet areas into dry areas and gradually lose discharge downstream will show decreases in the channel parameters and average velocity in a downstream direction. Over time, the channel shape, dimensions, and gradient adjust to accommodate the discharge normally passing through the channel.

9. When an area changes from being predominantly rural to largely urban, how is streamflow affected? (Box 10.1)

Impervious areas (pavement, roofs, etc.) vastly increase direct, stormwater runoff to streams and decrease the proportion of precipitation infiltrating the soil and bedrock; thus, following storms, discharges are raised substantially over what they would be in a natural (nonpaved) drainage basin. Also, the peak discharge arrives and passes downstream sooner than if the basin were in a natural condition. Base flow (the normal discharge not augmented by surface runoff) is reduced because less subsurface water is available to seep into the stream between storms.

10. Define base level. Name the main river in your area. For what streams does it act as base level? What is the base level for the Mississippi River?

Base level is the lowest elevation to which a stream can downcut or lower its channel (Figs. 10.8 & 10.9). The elevation of a major river at a junction with a tributary is the base level elevation (a temporary base level geologically) for the tributary. Dams and unusually hard bedrock layers function as temporary base levels for the upstream portion of the drainage basin. Sea level is the ultimate base level for rivers that discharge into the oceans; thus sea level is the base level for the Mississippi River. In closed, low-elevation basins such as Death Valley, CA, and the Jordan River Valley-Dead Sea depression, the lowest lake level or land surface elevation functions as base level for streams in the drainage basin.

11. Why do most streams have low gradients near their mouths?

Streams rise at higher elevations and flow toward lower elevations. Hill slopes are steeper in elevated areas and more gentle in areas at lower elevations. Thus for long river systems, headwater stream gradients are substantially steeper than those farther downstream toward the mouth of a river. A stream's gradient is always somewhat less steep than the slope of the land across which it flows.

12. Describe three ways in which a stream may erode its channel. Which one of these is responsible for creating potholes?

Stream channels are eroded by abrasion, scouring, and solution. Abrasion results from impacts of sediment particles with the bottom or with each other. Potholes (circular to elliptical, steep walled depressions in bedrock channels) are drilled by the abrasive action of sand and pebbles swirling round and round in turbulent eddies.

Scouring involves dislodging sediment particles from the channel walls and bottom and lifting them into the water column to be moved downstream. Sparingly soluble bedrock, such as limestone and dolostone, can be slowly dissolved by streams, especially if the water is initially acidic. Such a situation might arise if a stream originates in a marsh or swamp, or if acidic mine waters discharge into a stream.

13. If you were to collect a jar of water from a stream, what part of the load would settle to the bottom of the jar? What portion would remain in the water? What part of the stream's load would probably not be present in your sample?

The suspended load will eventually settle to the bottom, but the dissolved load will remain in solution in the clear water. Unless some sediment from the stream bottom was scooped up with the water, the bed load would not have been sampled.

14. What is settling velocity? What factors influence settling velocity?

Settling velocity describes the speed at which a particle, acted upon only by gravity, sinks through a motionless fluid, in this case water. We can assume that the density and viscosity of stream waters are essentially constant. In general, more massive particles settle faster than less massive ones; and, for spherical particles of equivalent diameters, settling velocities vary directly with density. Given equal masses, spherical and equidimensional particles settle faster than rodlike particles, and platelike particles such as mica flakes settle even more slowly. Tiny, clay-sized platelets settle so slowly that the slightest turbulence is enough to keep them in suspension.

15. Distinguish between capacity and competency.

These terms describe sediment transport characteristics of a stream. Competence describes or measures the maximum size of detrital particles (gravel, sand, etc.) that are moved by a stream. The largest particles in a stream move as bed load. Competency depends directly on velocity, so the largest particles are moved during flood stage when velocities are highest.

Capacity describes or measures the total amount or weight of sediment (bed, suspended, and dissolved loads) carried by a stream. The capacity is directly dependent on velocity and discharge, so substantially more sediment is moved during floods than during periods of low discharge. Large rivers with high discharges and low gradients have low competency and very high capacities; small mountain streams with steep gradients have high competencies (they can move boulders) but low capacities because they move a relatively small volume of sediment.

16. What factors promote the frequent flooding of broad areas along the Red River of the North (Box 10.2)?

Headwater streams of the Red River of the North join together to form the north-flowing Bois de Sioux River in the northeastern corner of South Dakota. Farther north, the name changes to the Red River of the North. These two rivers essentially form the entire boundary between North Dakota and Minnesota. The city of Grand Forks, ND, lies about 120 miles south the Canadian border; Fargo, ND, is upstream to the south, about halfway between Grand Forks and the South Dakota border. The Red flows northward into Canada and is joined by a major west-east flowing tributary, the Assiniboine River, at Winnipeg, Manitoba. From Winnipeg, the river flows northward and discharges into Lake Winnipeg. This large lake drains through a series of smaller lakes into the Nelson River which flows northward and eastward into Hudson Bay.

The flat floor of Lake Agassiz, a Pleistocene glacial lake, forms much of the land area in the Red River basin. The extreme flatness of the land and minimal channel incision guarantee that vast areas are inundated when the rivers and streams exceed their bankfull discharges. Also because of the land flatness, stream gradients are low. Essentially, the floodwaters form a vast, shallow lake as they slowly make their way downstream.

Climatic and seasonal factors can also contribute to flooding. For example, a heavy winter snowpack, frozen ground, and a sudden spring warm spell are a sure-fire formula for flooding. The bulk of the winter precipitation melts over a week or two, infiltration is nil because the ground is frozen, the stream and river channels can't carry the suddenly increased discharge, and flooding occurs. Slow gradual melting of the winter snowpack, of course, decreases the likelihood of flooding. In addition, the river may be frozen downstream (to the north) or jammed with floating ice. Both conditions impede flow and cause water to pond upstream, raising the potential for upstream flooding.

As early as February, predictions in print and visual media warned of potentially severe flooding when the heavy winter snowpack melted. The lead time easily exceeded the thirty-day waiting period required before insurance purchased under the National Flood Insurance Program goes into effect. Thus astute home and business owners could have bought insurance to protect themselves against the flooding. Most did not! An unusually heavy late winter snowfall only worsened the situation, and unfortunately, the spring thaw came quickly rather than slowly. Thus the spring, 1997, flooding was some of the worst seen in over a hundred years. A destructive fire in downtown Grand Forks provided a fitting climax to the flooding scene. The fire was burning out of control while the streets were covered with five feet of water.

17. Describe a situation that might cause a stream channel to become braided.

Braided channels result from excessive bed load. Glacial outwash streams (Fig. 10.15) are good examples. Rivers and streams that lose discharge downstream also typically become braided because they can no longer efficiently move bed loads acquired upstream where discharges and competence are higher. In addition, bed load influx from a highly competent, steeper tributary, an abrupt decreases in gradient, and an abrupt widening of the channel cross section can result in excessive bed loads and braiding.

18. Briefly describe the formation of a natural levee. How is this feature related to backswamps and yazoo tributaries?

Natural levees are mounds of sandy to silty sediment built up on floodplains directly adjacent to rivers and streams. When a stream is at flood stage, high velocities and turbulence allow silt and fine-sized sand to be carried in suspension. As sediment laden floodwaters spill onto the floodplain, velocities drop very quickly; and the coarser, suspended sediments (fine sand and silt usually) are deposited. With many successive floods, this sediment accumulates to form a natural levee. Thus on a broad floodplain, the highest ground is typically on the natural levee adjacent to the channel. Backswamps are the lower parts of the floodplain away from (or "back" from) the channel and natural levee. These areas stay inundated for longer periods following floods and receive less sediment (mostly clays and fine silt) than the natural levees; thus the backswamp areas remain at lower elevations than the natural levees.

Tributaries to a main stream with extensive, natural levees may flow for some distance parallel to the main stream before joining. These are called yazoo tributaries after the Yazoo River, a tributary to the lower Mississippi River in Mississippi.

19. In what way is a delta similar to an alluvial fan? In what way are they different?

Alluvial fans and deltas (Figs. 10.17 & 10.18) both represent accumulations of stream transported sediment at sites where gradients and velocities decrease abruptly. In an ideal sense, both show crude delta (�), map-view shapes and smaller channels (braided, anastomosing channels on alluvial fans and distributaries on deltas) that diverge outward from the apex of the delta or fan. Channel bottom and overbank sediments become progressively more fine grained in the downstream direction.

Deltas form at a land-water (sea or lake) interface where stream velocities decelerate to zero. Except for natural levees along major distributaries, deltas are very flat and close to sea level in elevation. Alluvial fans are entirely terrestrial and have no relation to sea level (elevations can be above or below); fan slopes are fairly steep. Alluvial fans, especially in their upper portions, are typically composed of much coarser sediments (gravels and coarse sands) than deltas (sands and finer-grained sediments). Although alluvial fan sediments may locally show fluvial cross bedding, they do not exhibit the large-scale, cross stratification that characterizes the whole deltaic accumulation. Finally, alluvial fans are products of weathering and erosion in dry lands with locally high relief resulting from active or recently active faulting. Deltas form where sediment laden rivers enter large bodies of water; no particular tectonic or climatic conditions are required.

20. Why does a river flowing across a delta eventually change course?

A delta is composed of soft and unlithified sediment deposited by the river; thus bank erosion and changes in the channel come about easily. At any one time in the evolution of a delta, the bulk of the sediment is carried along the major distributaries.

As the sediment is deposited, new land is built, and the distributary channels extend farther out into the lake or sea; thus gradually their gradients are lowered. Other distributaries and potential distributaries follow shorter routes to the sea (or lake); thus they have higher gradients. Eventually discharge is diverted into existing or newly-cut distributaries with steeper gradients and the lower-gradient distributaries are abandoned. Thus without man's intervention, much of the Mississippi's discharge might have already been diverted into the Red River-Atchafalaya outlet, bypassing the present-day channel from north of Baton Rouge to the Gulf of Mexico (Fig. 10.20).

21. Each of the following statements refers to a particular drainage pattern. Identify the pattern.

(a) Streams diverging from a central high area such as a dome. - radial

(b) Branching, "treelike" pattern. - dendritic

(c) A pattern that develops when bedrock is crisscrossed by joints and faults.

- rectangular

22. Describe how a water gap might form. (Box 10.3.)

Water gaps (Figs. 10.D & 10.E, Box 10.3; Fig. 10.34) are common features in mature landscapes formed on tilted or folded strata with varying resistances to weathering and erosion. The Valley and Ridge Province of the Appalachians is a good example. With the passage of time, a trellis drainage pattern develops as the landscape is lowered by erosion. Long, tributary streams to the "master river" erode linear valleys into outcrop areas of weak, easily eroded strata; linear ridges develop on outcrop areas of the harder strata. However, the courses of the main (master) rivers in such a region are superposed across the weaker and harder strata alike, their positions being inherited from a time before the valley and ridge topography was formed. Thus a water gap describes the short, steep-sided valley segment or gap through which the master stream flows across the outcrop area of harder, ridge-forming strata.

23. List and briefly describe three basic flood-control strategies. What are some drawbacks of each?

Three engineering strategies are channelization, construction of levees, and construction of dams.

Channelization describes an engineering activity wherein a natural stream/river channel is straightened (meander loops are bypassed), freed of obstructions (fallen trees, large boulders, etc. are removed), widened in many cases, and smoothed. Thus the modified channel has a steeper gradient and lower roughness factor than the natural channel. For a given discharge, these changed channel characteristics all contribute to increased average velocities in the modified channel over those in the natural channel. The increased average velocity is accompanied by lower water surface elevations for a given discharge, thus providing for lower flood peaks in the modified channel.

Channelization has the effect of exporting floods to unmodified channel reaches farther downstream. When the "modified, faster-moving discharge" arrives at the unmodified channel reach, the lower gradient, smaller cross section, and rougher channel are reestablished. Now, however, the natural channel can't efficiently handle the rapidly arriving discharge, so the water "piles up" and flood stage elevations are higher than they would have been if the whole length of the channel had been left unmodified and the natural mechanisms for upstream flooding and overbank storage were unimpeded.

Channelization also has a not-so-sterling record of unwanted channel erosion (review the Blackwater River, MO, example in the text; see also the Blackwater River case history, p. 126, Environmental Geology, seventh edition, Keller, E. A., Prentice Hall), degraded water quality and aquatic ecology, loss of riparian habitat, and loss of pleasing, esthetic qualities associated with natural streams. Channelized streams also can deliver excessive nutrients and toxic pollutants to downstream locations farther and faster than would be the case for a stream with a natural, unmodified channel.

Artificial levees function in essentially the opposite way from channelization. Levees confine the water to the channel and prevent it from moving onto the floodplain. Thus for a given discharge, river-stage elevations are raised, not lowered. The discharge is all confined to the cross sectional area of the channel and the floodplain does not function to "store" excess water during floods and to release it slowly as the river stage declines following the flood. Artificial levees export floods upstream, downstream, and to adjacent parts of the river that have no levees. Thus for a given discharge, flood-stage elevations are raised in upstream and adjacent locations without levees. Downstream, the unleveed natural channel can't efficiently convey the fast-moving discharge delivered from the levee-lined channel, so as with channelized streams, higher flood stages than expected for a given discharge are observed. Levee failures result in sudden, catastrophic floods that are far more threatening to lives and property than would be the case for a "natural flood" in an unleveed floodplain. These observations were well-documented during the Upper Mississippi basin floods of 1993 (Figs. 10.35 & 10.36).

Dams and reservoirs are the mainstay engineering modifications that allow streams and rivers to be managed for electrical power generation, water supply, flood control, and navigation. Upstream flood-stage discharges can temporarily be stored in reservoirs and released slowly. Thus downstream discharges are lowered and associated peak flood-stage elevations are lowered or eliminated entirely. Flood control, however, may not be the number one management consideration. For example, reservoir managers may "bet wrong" by discounting the possibility of several late summer/early fall heavy rainfall events and store runoff from a major early summer storm to insure for an adequate supply of water during the normally drier months. If unexpected late rainfall events materialize or if an unusually long period of above normal precipitation occurs, the reservoir or reservoirs are filled or nearly filled and have incidental excess storage capacity if any. At these times, the reservoir managers have no choice but to open the floodgates and hope for the best. Above all other considerations, a worst-nightmare-failure-of-the-dam scenario has to be prevented! Thus a filled reservoir pool has no flood control value, and massive releases from the dam may cause or intensify downstream flooding.

Dams also trap sediment leading to possible adverse environmental effects downstream and sooner or later, reducing the water storage capacity of the reservoir. Small reservoirs can lose much of their storage capacity to sedimentation in a short time. A very large reservoir, such as Glen Canyon (Lake Powell), is estimated to lose about half its initial storage capacity in a few hundred years.

Downstream effects may include scouring and deepening of the channel, loss of natural soil replenishment during flooding, and severe erosion and loss of delta wetlands, such as has occurred in the Nile delta since closing of the Aswan High Dam. Well-intentioned flood control dams on small rivers and streams in areas with rugged coastal topography, such as southern California, will trap so much sediment that local beaches are "starved of sand" and beach erosion occurs.

Flood-management efforts are shifting more toward responsible land use and zoning regulations to reduce threats, damage, and costs of flooding. The writer watched an interesting TV interview with the director of FEMA, the Federal Emergency Management Agency. This agency has taken a leading role in assisting victims during the post-disaster cleanup and recovery phases and is strongly promoting the National Flood Insurance Program. His hosts adopted an adversarial position and asked why the average U. S. taxpayer should shell out to make subsidized insurance available for individuals with private residences or commercial assets in flood-prone areas. His response was very enlightening to all taxpayers, including earth scientists. Pay up; provide the insurance. Otherwise, you will foot the total bill for assistance and recovery, not just the total minus the amount paid in flood insurance premiums. This situation clearly provides strong incentives for a national flood-response policy based on rational scientific principles and common-sense land use regulations.