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3.6: Classifying Rivers - Geosciences

3.6: Classifying Rivers - Geosciences


Rivers are varied in so many ways that you should expect complexity in classification. Rivers can be classified in several ways:

• by the nature of their substrate
• by the percentage of time they flow
• by their relationship to the groundwater table
• by the kind of sediment load they carry
• by the dominant particle size of the bed sediment • by their morphology

In the following I’ll make some comments on classification of rivers in each of these ways. (But I’ll defer discussion of classification of rivers by sediment load until later, after I’ve talked about the sediment load.)

Nature of Substrate:

Some rivers, especially small rivers in mountainous areas, flow directly on bedrock. Such rivers are called bedrock rivers, or non-alluvial rivers (Figure 3- 22A). Other rivers, especially large rivers, flow on a bed a sediment they have deposited and can continue to transport. Such rivers are called alluvial rivers (Figure 3-22B). Of course, some rivers lie in between, in that they have bedrock beds in some reaches and alluvial beds in other reaches (Figure 3-23). Alluvial rivers have held most of the problems, fascination, and importance for fluviologists, but if you’re a white-water canoeing enthusiast I suppose you’re more interested in bedrock rivers.

Relationship to Groundwater Table:

Think about the relative position of the water surface in the river and the local groundwater table. If the water surface in the river lies above the local groundwater table in the river banks, then the river loses water to its banks. Such a river is said to be an effluent river (Figure 3-24A). On the other hand, if the water surface in the river lies below the local groundwater table in the banks, then the river gains water from its banks. Such a river is said to be influent river (Figure 3-24B). (These two terms are difficult to keep straight. It helps to think in terms of alternative terminology: an effluent river is also called a gaining river, because it is gaining water from the adjacent substrate, and an influent river is called a losing river.) Keep in mind that it’s also possible for the groundwater table to lie entirely below the river bed (Figure 3-25). The river is still called an effluent river in that case.

Percentage of Time the River Flows:

Some rivers show a flow of water all the time, even long after the last rainstorm in the watershed. Such a river is called a perennial stream. Other rivers flow for only a short time after a rainstorm, and for the rest of time, usually most of the time, their beds are dry. Such a river is called an ephemeral stream. Some rivers lie between these two extremes: during the wetter part of the year they flow as a perennial stream, whereas during the drier part of the year they flow as an ephemeral stream. Such a river is called an intermittent stream. Figure 3-26 shows cartoon hydrographs of a perennial stream, an ephemeral stream, and an intermittent stream.

In an ephemeral stream, the water table always lies below the bed of the stream; the stream never receives any water from its bed or banks. In a perennial stream, the situation is more complicated. Think about the relationship between the river level and the groundwater table in some time period that starts in a dry spell, extends through a major rainfall event in the watershed, and ends during another dry spell. At the end of the first dry spell the river level lies below the groundwater table in the river banks (Figure 3-27A). After a heavy rainfall the river stage rises rapidly to lie well above the level of the groundwater table in the banks (Figure 3-27B). Groundwater is stored in the river banks, in the sense that the groundwater table is locally and temporarily higher there than in the surroundings. At the end of the rainy period both the river stage and the groundwater level are of about the same height and are about at their highest (Figure 3-27C). Then (Figure 3-27D) both the river stage and the groundwater table fall back to the dry-spell situation shown in Figure 3-27A. This sequence of events is called the runoff cycle.

Morphology:

The morphology of rivers, especially in plan view, varies enormously. The most common way to classify rivers is on the basis of their plan-view morphology. The morphology of rivers is bound up in a complex way with the nature of the sediment load, so a full appreciation of this section must await Section 8, on the sediment load of rivers.

Two characteristics are used in the classification of rivers by morphology: sinuosity and “multichanneledness”. Sinuosity can be defined with respect to two arbitrary points along the river as the ratio of the along-channel distance between the two points and the straight-line distance between the points (Figure 3-28). The minimum sinuosity, for a straight river, is 1; the more sinuous the river, the greater its sinuosity. Very sinuous rivers can have values of sinuosity approaching 4. Multichanneledness, an awkward but useful word, reflects the number of individual flow channels shown by a river in a cross-stream traverse across the entire river system. Many rivers have only one channel, except perhaps where an occasional island divides the channel into two. Other rivers show a large number of channels, all of about the same size and nature, separated by numerous bars and islands. The individual channels of such a river are called anabranches.

Sinuosity and multichanneledness are to a large extent independent of one another, so it’s natural to resort to a two-independent-variable pigeonhole classification with sinuosity along one axis and multichanneledness along the other axis (Figure 3-29). Straight rivers—those with sinuosity not much greater than 1—are surprisingly uncommon in nature. In fact, it’s hard to keep rivers straight: humans straighten them out for their own purposes, and the rivers try to become sinuous again, by erosion and deposition on the banks. Both braided rivers (low-sinuosity, multichanneled) and meandering rivers (high sinuosity, single-channeled) are very common; more on them later. Anastomosing rivers (high-sinuosity, multichanneled) are much less common.


Substantial proportion of global streamflow less than three months old

Biogeochemical cycles, contaminant transport and chemical weathering are regulated by the speed at which precipitation travels through landscapes and reaches streams 1 . Streamflow is a mixture of young and old precipitation 2 , but the global proportions of these young and old components are not known. Here we analyse seasonal cycles of oxygen isotope ratios in rain, snow and streamflow compiled from 254 watersheds around the world, and calculate the fraction of streamflow that is derived from precipitation that fell within the past two or three months. This young streamflow accounts for about a third of global river discharge, and comprises at least 5% of discharge in about 90% of the catchments we investigated. We conclude that, although typical catchments have mean transit times of years or even decades 3 , they nonetheless can rapidly transmit substantial fractions of soluble contaminant inputs to streams. Young streamflow is less prevalent in steeper landscapes, which suggests they are characterized by deeper vertical infiltration. Because young streamflow is derived from less than 0.1% of global groundwater storage, we conclude that this thin veneer of aquifer storage will have a disproportionate influence on stream water quality.


Geoscience Course Listings

There are many ways to satisfy your 7 units of Science general education distribution requirements. Typically, this is one lecture class (3 units) and one lecture + lab class (4 units). Beginning with Fall 2011, in addition to GEOG 103: Physical Geography of Earth’s Environment lecture (DE or classroom), we will also be offering the accompanying lab (GEOG 104) online. You can take the all in one class, all online, or a hybrid. Additionally, we will be offering GEOL 101: Exploring Planet Earth lecture as a distance education class.

View course descriptions below, and register to satisfy your Science requirements with the flexibility of an online course!

Coming soon, Spring 2012:
Two new distance education physical science labs: Distance Education GEOL 100: Natural Disasters + lab, and GEOL 101: Exploring Planet Earth lab.

Undergraduate Level Courses (Geology)

Undergraduate Level Courses (Geography)

GEOG 103
Physical Geography of Earth’s Environment
(3 cr)
Introduction to the processes that influence weather, rivers, oceans, climate, deserts, glaciers, and their associated ecosystems. Emphasizes relationships between humans and our environment. Satisfies the General Education Core requirement for a science course. *Lecture may be combined with optional lab (GEOG 104), which satisfies General Education Core requirement for a laboratory science course.
GEOG 104
Physical Geography Lab

(1 cr)
Provides an opportunity to apply concepts in physical geography, including map interpretation, computer GIS, meteorological processes, development of landforms, and an understanding of the dynamics of the earth. *Corequisite: GEOG 103
GEOG 116
Introduction to Oceanography

(3 cr)
Fundamentals of oceanography will be covered including a brief history followed by the spatial aspects of geological, physical, chemical and biological oceanography. An emphasis will be placed on the role of oceans on climate change in the past, present and future, including global warming. *Lecture
GEOG 390
Meteorology and Climatology

(3 cr)
Study of the atmosphere and its effect on our daily weather. Horizontal and vertical currents in the atmosphere and the distribution of solar energy, moisture, and storms. *Lecture. Prerequisites:
GEOG 103 MATH 128

GEOL 735
Seminar in Environmental Geology

(3 cr)


An overview of the hydrology of non-perennial rivers and streams

Margaret Shanafield, College of Science & Engineering, Flinders University, Adelaide 5001, Australia.

Contribution: Conceptualization, ​Investigation, Writing - original draft, Writing - review & editing

School of Earth Sciences, University of Western Australia, Crawley, Australia

Contribution: Formal analysis, Writing - original draft, Writing - review & editing

Earth and Planetary Sciences, University of California, Santa Cruz, California, USA

Contribution: Conceptualization, ​Investigation, Writing - original draft, Writing - review & editing

School of Geosciences, University of Louisiana, Lafayette, Louisiana, USA

Contribution: Conceptualization, ​Investigation, Writing - original draft, Writing - review & editing

College of Science & Engineering, Flinders University, Adelaide, Australia

Margaret Shanafield, College of Science & Engineering, Flinders University, Adelaide 5001, Australia.

Contribution: Conceptualization, ​Investigation, Writing - original draft, Writing - review & editing

School of Earth Sciences, University of Western Australia, Crawley, Australia

Contribution: Formal analysis, Writing - original draft, Writing - review & editing

Earth and Planetary Sciences, University of California, Santa Cruz, California, USA

Contribution: Conceptualization, ​Investigation, Writing - original draft, Writing - review & editing

School of Geosciences, University of Louisiana, Lafayette, Louisiana, USA

Contribution: Conceptualization, ​Investigation, Writing - original draft, Writing - review & editing

Funding information: National Science Foundation, Grant/Award Number: DEB-1754389

Abstract

Non-perennial rivers and streams are ubiquitous on our planet. Although several metrics have been used to statistically group or compare streamflow characteristics, there is currently no widely used definition of how many days or over what reach length surface flow must cease in order to classify a river as non-perennial. At the same time, the breadth of climate and geographic settings for non-perennial rivers leads to diversity in their flow regimes, such as how often or how quickly they go dry. These rivers have a rich and expanding body of literature addressing their ecologic and geomorphic features, but are often said to be ignored by hydrologists. Yet there is much we do know about their hydrology in terms of streamflow generation processes, water losses, and variability in flow. We also know that while they are prevalent in arid regions, they occur across all climate types and experience a diverse set of natural and anthropogenic controls on streamflow. Furthermore, measuring and modeling the hydrology of these rivers presents a distinct set of challenges, and there are many research directions, which still require further attention. Therefore, we present an overview of the current understanding, methodologic challenges, knowledge gaps, and research directions for hydrologic understanding of non-perennial rivers critical topics in light of both growing global water scarcity and ever-changing laws and policies that dictate whether and how much environmental protection these rivers receive.

Abstract

The hydrology of non-perennial rivers is characterized by variability in streamflow generation and loss this poses challenges to how we measure and model these diverse river systems.


Supplementary Information

Supplementary methods including Figs. 1–13 and Table 1.

Supplementary Data 1

Main dataset. The table is structured so that each row is a valley network and columns include the ID number, valley name (if applicable), latitude/longitude, and the six metrics and their respective error.

Supplementary Data 2

This table includes two sheets. The first is the table of parameters, where each row is a parameter, and columns are the parameter symbol, definition, units, values (lower, average, and upper bounds), and references. The second sheet contains the metric predictions (upper, average, and lower values).

Supplementary Data 3

PCA classification and confidence results of the study. Rows correspond to valley networks, whereas columns give their ID, name, latitude/longitude, the distances to each of the synthetic valley network erosional groups, the relative distances, distances minus statistical threshold, and the final classification result (1 is fluvial, 2 is glacial, 3 is sapping, 4 is subglacial, 5 is undifferentiated). The last column, confidence, goes from highest (1) to lowest (4).

Supplementary Data 4

Longitudinal profile observations and undulation interpretations. Rows correspond to valley networks, columns are valley network ID, name, and a description of the longitudinal profile undulations and interpretations.

Supplementary Data 5

Sensitivity analysis for the principal component results. The first sheet contains the sensitivity analysis summary, columns are the sample size and the five metrics. The second sheet contains a total of 35 individual analysis for different sample sizes, as indicated.


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