Drainage Basin Morphometry | Geology Optional Notes for UPSC PDF Download

Morphometric Analysis of River Basins

• Morphometry is the measurement of the external form of landforms, while analysis involves detailed evaluation. In the realm of Geomorphology, Morphometric Analysis refers to the meticulous assessment of landforms through mathematical measurement.
• Quantitative measurement through mathematical tools aids in the precise analysis of landforms for planning and developmental purposes. It is a crucial method as it quantifies the significant features of landforms that have evolved over time.

Morphometric Analysis of River Basins

• River basins serve as important units for Morphometric Analysis, aiding in the comprehension of geomorphic and hydrologic processes. A river basin refers to a land area where surface water infiltrates from various directions and sources, ultimately converging at a single point before exiting the basin. Upon exiting, the water typically merges with another body of water, such as a river, lake, sea, or ocean.
• These basins are crucial ecological entities for the effective management and planning of natural resources due to the presence of a singular outlet for all materials like soil and water within the entire area. Morphometric Analysis of river basins proves highly beneficial for management and planning endeavors, offering precise insights through mathematical computations.
• The Morphometric Analysis of a river basin encompasses three primary dimensions:
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  • Linear Aspects: This dimension focuses on one-dimensional features related to the river basin.
  • Areal Aspects: These aspects pertain to two-dimensional characteristics of the river basin.
  • Relief Aspects: These aspects involve the analysis of three-dimensional attributes within the river basin.

By conducting Morphometric Analysis within these three dimensions, researchers and planners can gain a comprehensive understanding of the river basin's structure, behavior, and functions. For instance, when analyzing the linear aspects, parameters like stream length and drainage density are considered. Areal aspects may involve the study of basin area, shape, and relief ratio. Relief aspects, on the other hand, delve into the examination of elevation, slope, and relief amplitude.This analytical approach aids in effectively managing water resources, predicting flood patterns, and designing sustainable development strategies tailored to the specific characteristics of the river basin.

Linear Aspects of Morphometric Analysis

Stream Order

• Stream Order involves arranging streams in a river basin hierarchically based on their size and flow patterns.
• Strahler's method, widely used, designates the smallest streams as 1st order, with higher orders formed as streams merge.
• Key points to remember when designating stream orders:
  • Do not rely on mathematical calculations.
  • When two streams of the same order converge, the resultant stream is one order higher.
  • When different-order streams join, consider the higher order.
• Transition from lower to higher order streams brings changes like:
  • Decrease in water flow velocity.
  • Increase in stream width and water volume.
  • Rise in stream water temperature.
  • Higher sediment load and turbidity.
  • More mineral nutrients present.
  • Shift from rocky to muddy/sandy stream bottom.

Stream Number

• The Stream Number refers to the total count of streams within each order in Morphometric analysis.
• It is the second step following Stream Order determination.
• As stream order increases, the Stream Number decreases, showing an inverse relationship.
• Horton formulated the 'Law of Stream Number,' indicating that plotting Stream Number (arithmetically) against Stream Order (logarithmically) results in a negative linear trend.
• This relationship implies that streams of varying orders in a basin often form a Geometric Series.
• For instance, in a hypothetical 6th-order river basin, stream numbers would ideally follow a sequence like 1, 3, 9, 27, 71, and 213.
• In practical scenarios, such as a 4th-order river basin, the stream numbers might deviate from the ideal sequence, like 1, 5, 14, and 62, as illustrated in a logarithmic scale in a given figure.

Understanding Bifurcation Ratio in River Basins

• The Bifurcation Ratio is a crucial concept in hydrology that helps us understand the relationship between the number of streams of one order and the next higher order.
• It is calculated by dividing the total number of streams in a particular order by the total number of streams in the next higher order.
• For example, if we consider the streams in the 1st order and compare them to those in the 2nd order, we can calculate the Bifurcation Ratio.

Mean Bifurcation Ratio and Its Significance

• The Mean Bifurcation Ratio is obtained by averaging all the bifurcation ratios of consecutive streams in a river basin.
• It provides insights into how the number of stream segments increases as we move from higher to lower orders in a river system.
• Essentially, it represents the constant of the Geometric Series discussed under the Stream Number concept.

Factors Influencing Bifurcation Ratio

• The Bifurcation Ratio is affected by various factors such as relief, rock type, and the dissection of rocks in the area.
• In regions with relatively uniform rock types, the Mean Bifurcation Ratio typically falls between 3 and 5.
• However, if the drainage pattern is influenced by geological structures, the Mean Bifurcation Ratio can exceed 5.

Implications of Bifurcation Ratio

• A low bifurcation ratio can lead to increased flooding risks as water tends to collect rather than disperse evenly.
• Human activities can impact the bifurcation ratio, potentially heightening the risk of flooding within a basin.
• In areas with consistent climate, rock types, and geological history, the Bifurcation Ratio tends to remain relatively constant from one order to the next, serving as a defining characteristic of the entire basin.

Stream Length and Related Concepts

• Stream Length: Total length of all streams of a particular order.
• Mean Stream Length: Average length of streams of a specific order.
• Relationship with Stream Order: More streams with lower order lead to increased total stream length.
• Mean Stream Length Trend: Decreases as stream order decreases.
• Stream Length Ratio: Ratio between mean length of streams of a specific order and the next lower order.
• Stream Length Ratio Equation: SLR = Ln / Ln-1

Table Overview

• Stream Order: Represents the order of the stream.
• Stream Number: Number of streams in a particular order.
• Bifurcation Ratio: Ratio between streams in successive orders.
• Mean Stream Length: Average length of streams in a specific order.
• Stream Length Ratio: Ratio between mean lengths of streams in consecutive orders.
• Average Basin Area: Mean area drained by a river.
• Average Channel Slope: Average slope of the river channel.

Table Data Interpretation

• Stream Order 1: 24.4 streams with a Bifurcation Ratio of 3.6 and a Mean Stream Length of 1.8 km.
• Stream Order 2: 14.2 streams with a Bifurcation Ratio of 1.08 and a Mean Stream Length of 1.19 km.
• Stream Order 3: 5.5 streams with a Bifurcation Ratio of 1.29 and a Mean Stream Length of 3.79 km.
• Stream Order 4: 14.9 streams with a Mean Bifurcation Ratio of 4.07 and a Mean Stream Length of 6.1 km.

Note: The data indicates trends in stream characteristics based on stream order and provides insights into stream network development.

Law of Stream Length by Horton

• When we look at stream lengths across different stream orders, we calculate the Stream Length Ratio. This ratio compares the mean length of streams in one order to that of the next lower order. For instance, we compare the mean length of 4th order streams to that of 3rd order streams, and so forth.
• Horton, similar to his Law of Stream Number, introduced the Law of Stream Length. According to this law, the Cumulative Mean Length of streams increases exponentially as we move to higher stream orders. If we plot stream order on the X-axis on an arithmetic scale and the cumulative mean length of streams on the Y-axis on a logarithmic scale, we observe a positive linear trend.

Relationships between Stream Order and Stream Length

• There exists a negative correlation between Stream Order and Total Stream Length. This means that as stream order increases, the total length of streams decreases.
• Conversely, there is a positive correlation between Stream Order and Mean Stream Length. As the stream order increases, the average length of individual streams tends to increase.
• Moreover, there is a positive correlation between Stream Order and Cumulative Mean Stream Length. This relationship is particularly intriguing as the cumulative mean stream length increases exponentially with each higher stream order.

By understanding these relationships, we can gain insights into how streams evolve and grow in length as they progress through different orders. Let's explore these concepts further with some illustrative examples:Imagine a small mountain with streams flowing down its slopes. As these streams merge and form higher-order streams, we observe a pattern where the average length of streams increases. This increase in length with higher stream orders showcases the principles of the Law of Stream Length by Horton.In a practical scenario, consider a river system where we compare the lengths of tributaries at different orders. The Law of Stream Length helps us predict how stream lengths change as we move from smaller to larger stream orders. This understanding is crucial for studying river networks and their characteristics.

Overview of Sinuosity Index

• Sinuosity Index helps us understand how much a river meanders.
• It compares the actual length of a river to the straight-line length it would have.

Calculation of Sinuosity Index

• Sinuosity Index (SI) = Actual length of the stream / Expected straight path
• It is used to study the impact of terrain on river flow patterns.

Schumm's Categorization

• Schumm's method categorizes rivers based on their Sinuosity Index values.
• For example, a Sinuosity Index of 1 indicates a straight river course.
• A Sinuosity Index over 20 signifies a highly meandering river.

Types of River Courses

• Straight course: Sinuosity Index = 1
• Transitional course: Sinuosity Index between 1 and 20
• Regular course: Sinuosity Index in a moderate range
• Irregular course: Sinuosity Index with significant meandering

Example and Interpretation

• If a river has a Sinuosity Index of 1.17, it falls in the transitional category.
• This means the river's course is between straight and moderately meandering.

Areal Aspects of Morphometric Analysis

Basin Shape

• The shape of drainage basins varies depending on factors like relief, rock type, slope, and geological structure.
• Ideal basin shape resembles a pear, with mountainous zones often having more elongated basins compared to streams from hilly regions to plains.
• Assessment of basin shape helps in understanding hydrological processes.
• Schumm's Elongation Ratio is a popular method to assess basin shape, expressed as ER = D / L.
• ER (Elongation Ratio) is calculated based on the diameter of a circle with the same area as the basin (D), basin length (L), and basin area (A).
• ER ranges from 0 to 1, with a higher value indicating a more circular basin and a lower value showing a more elongated shape.
• For example, if a river basin has an area of 44.2 km2 and a length of 4.9 km, with an ER of 0.76, it suggests a more circular, pear-shaped basin.

Basin Area Concepts

• Basin Area is the total area of a stream of a specific order within a drainage system.
• Mean Stream Area refers to the average stream area of a particular order.
• The Basin Area increases as the stream order increases.

Law of Basin Area

• Stahler's Law of Basin Area states that the mean basin areas of higher stream orders form a geometric series.
• Plotting Mean Basin Areas against respective basin orders results in a straight line on a logarithmic scale.
• This relationship is depicted in graphical representations like Figure 5.

Basin Area Ratio

• Basin Area Ratio (BAR) is the ratio between the mean area of streams of a given order and the mean area of streams in the previous lower order.
• The formula for calculating BAR is BAR = An / An-1.

Calculation of Basin Area Ratios

• BAR can be determined between streams of different orders, such as 4th order and 3rd order, 3rd order and 2nd order, and 2nd order and 1st order.

Drainage Frequency

• Drainage Frequency refers to the total number of streams per unit area.
• It demonstrates the cutting up of a relatively flat landscape through stream erosion and incision.
• First and second-order streams in various regions are often seasonal, forming during rainy seasons due to heavy rain and turning dry afterward.
• These seasonal streams can resemble gullies, which deepen and widen with subsequent rainy seasons, a process known as dissection.
• Higher Drainage Frequency indicates lower permeability and infiltration levels.

Factors Influencing Drainage Frequency

• Drainage Frequency is influenced by variations in rock structure within a basin.
• Younger topographies typically exhibit a higher number of streams compared to mature topographies.

Calculation and Representation

• The drainage basin is divided into small grid cells of equal area for determining Drainage Frequency.
• Stream count per cell is divided by the area of the cell to obtain Drainage Frequency.
• To visualize the spatial pattern, the grid cell values are categorized into different classes for creating a choropleth map.

Categories of Drainage Frequency

• Categories include Very Poor, Poor, Moderate, High, and Very High, based on the stream density in a given area.

Understanding Drainage Density

• Definition: Drainage Density is a crucial characteristic of a drainage basin that impacts the drainage system's structure. It is determined by factors such as geology, climate, and terrain type.
• Influence of Climate: In regions with high humidity, areas with significant elevation differences exhibit a higher drainage density compared to regions with lower relief levels.
• Geological Impact: Drainage density tends to be higher in areas with impermeable but easily erodible rocks like clay.
• Effect on Water Flow: During storms or sudden heavy rainfall, water flows more rapidly in channelized streams than over the land surface, reducing the risk of flash floods in regions with high drainage density.
• Calculation: Drainage density is calculated as the ratio of the total length of all stream channels in a basin to the total area of the basin using the formula Dd = L1 * L2 * L3 * ... * LN / A.
• Spatial Variation: To understand the spatial distribution of drainage density within a basin, the area can be divided into equal grid squares. The drainage density of each square is computed, and a choropleth map can be created based on these values.

Relief Aspects of Morphometric Analysis

• Stream Slope:
  • Streams flow on slopes where water velocity depends on the slope.
  • Stream slope is the ratio of vertical drop to horizontal distance.
  • Vertical drop is calculated by the difference in the stream's relief at its origin and mouth.
  • Horizontal distance is measured between the stream's origin and mouth.
  • Mean Stream Slope of any stream order is the average slope of that order.
  • The equation for Stream Slope and Mean Stream Slope is SS = V / H, where SS is Stream Slope, V is Vertical drop, and H is Horizontal distance.
  • MSS is calculated as the summation of vertical drop of the nth order stream divided by the number of nth order streams, over the summation of horizontal distance of the nth order stream divided by the number of nth order streams.
  • Horton formulated the Law of Stream Slope, stating that Mean Stream Slope increases with decreasing stream orders in geometric progression.
  • When Mean Stream Slope is plotted against basin orders, a negative relation in the form of a straight line is observed.

By understanding the concept of stream slope and mean stream slope, we can analyze how water bodies like rivers and streams interact with the landscape. This knowledge helps in various fields like hydrology, geography, and environmental science.

River Profile Overview

• The river profile provides a vertical snapshot of the river's course, showcasing its features in two main ways: along the river (Long Profile) and across the river (Cross Profile).

Longitudinal Profile (Long Profile)

• A Longitudinal Profile is a graph that displays the relationship between distance and elevation along the river's course.
• It illustrates changes in altitude from the source to the mouth of the river, typically showing a concave shape.
• The slope of the Longitudinal Profile tends to become gentler as the river approaches its mouth.
• River profiles experiencing rejuvenation may exhibit pronounced breaks, indicating nick points or heads of rejuvenation.

Cross Profile

• A Cross Profile is a graph that represents the relationship between distance and elevation across the width of the river.
• These profiles can be drawn at various points along the river's course.
• In the upper course, the Cross Profile often appears V-shaped, changing in shape as the river progresses.
• Both Longitudinal and Cross Profiles offer insights into the underlying materials, geologic processes, and the geomorphic history of a region.

Area-Height Analysis in River Basins

• Area-Height analysis, also known as Hypsometric analysis, involves studying the relationship between altitude and basin area to assess erosion levels and dissection stages in a river basin.
• Two methods are commonly used to draw hypsometric curves in a river basin to compare erosion levels:
  • In the first method, contour values are plotted against the percentage of surface area above that contour. A more concave curve indicates a more mature river.
  • The second method involves plotting relative height (h/H) on the Y-axis and relative area (a/A) on the X-axis. This method helps compare different river basin areas by analyzing the shapes of the curves.
• Hypsometric curves reflect different river stages like Youth, Mature, and Old, enabling comparisons between denudation stages of different river basins.
• Morphometric Analysis, a quantitative study of the earth's external features, is crucial for understanding river basins due to its simplicity and utility in planning purposes as an ecological unit.
• Twelve significant parameters of morphometry are discussed under linear, areal, and relief aspects, with detailed explanations and examples provided to illustrate their importance.