Changes in River Characteristics

• Long profile — a side-on view showing how the altitude of a river changes from its source to its mouth. It is typically a smooth, concave-up curve: steep near the source, flattening towards the mouth.

• Cross profile — a cross-section across the river valley at a particular point, showing the SHAPE of the valley and channel (V-shape, U-shape or flat-floored).

• Gradient — DECREASES from source to mouth (the long profile becomes less steep).
• Channel width — INCREASES from source to mouth (more water needs a wider channel).
• Sediment size — DECREASES from source to mouth (attrition chips away corners and breaks particles into smaller, more rounded fragments).

It seems strange because a steeper gradient should give faster flow — and in the upper course the gradient IS steeper. The reason velocity still rises downstream is channel efficiency.

In the upper course, the channel is narrow with rough, irregular sides and a boulder-strewn bed. A large proportion of the water is in contact with the channel boundaries, so friction wastes most of the river's energy. The water also has to navigate obstacles like waterfalls and rapids. Net result: average velocity is LOW despite the steep slope.

In the lower course, the channel is much wider, deeper and smoother (the bed is now alluvium, not boulders). Most of the flow runs away from the boundaries, so friction acts on a smaller proportion of the water. With less energy lost to friction, average velocity is higher even though the gradient is gentler.

Description. In the upper course, the cross profile is a narrow V-shape: a small channel in the base of a steep-sided, narrow valley. In the lower course, the cross profile is a wide, flat-floored valley with a much larger meandering channel and a broad floodplain on either side.

Reason. Lateral erosion in the lower course migrates meanders sideways across the valley floor, while regular over-bank flooding deposits silt and clay to build up a wide flat floodplain. Together these processes WIDEN the valley and flatten its base — producing the open, flat-floored shape instead of the narrow V of the upper course.

As a river flows from source to mouth, discharge increases sharply because tributaries continually add water — by the lower course the discharge is many times that at the source.

To convey this growing discharge, channel width and depth both increase. The channel becomes larger and (because the bed is now alluvium, not rough boulders) smoother. This larger, smoother channel is more efficient: a smaller fraction of the water is in contact with the bed and banks, so less energy is lost to friction. Even though gradient decreases — the long profile flattens towards the mouth — average velocity actually INCREASES.

Sediment changes too. In the upper course, weathering and mass movement supply large, angular fragments from valley sides; the river adds these to its bedload. As particles travel downstream, attrition (particles colliding with each other) chips away corners and breaks them into smaller pieces. By the lower course, sediment is small and well rounded — fine silt and clay dominate, carried in suspension and solution.

These characteristics are linked into a coordinated system: rising discharge demands a bigger, smoother channel, which raises velocity; rising transported distance produces finer rounder sediment; falling gradient is offset by greater channel efficiency. The Bradshaw model (1978) summarises these coordinated trends.

Named river: the Ganges (India / Bangladesh). The Ganges flows ~2,525 km from the Himalayas to the Bay of Bengal and shows the classic source-to-mouth signature for all three variables.

Channel cross-section. Near the source at the Gangotri Glacier (~4,100 m altitude), the channel is narrow (~5-15 m wide, <2 m deep), cut into hard, weathered Himalayan rock. The cross-profile is a steep V-shape. As the river descends through Uttarakhand and into the plains around Haridwar, the channel widens dramatically (~150-300 m). At Allahabad / Prayagraj the Yamuna joins the Ganges, doubling the discharge and producing channels several hundred metres wide. By the time the river reaches Bangladesh and splits into the deltaic distributaries (Padma + Meghna), channels can be over 1 km wide in flood. The cross-profile transitions from V-shape → wider open valley → very wide flat-bottomed valley with extensive floodplain.

Velocity. Counter-intuitively, velocity INCREASES downstream despite the falling gradient. In the Himalayan headwaters the gradient is very steep (~10-50 m/km) but the rough boulder bed wastes most of the energy in friction; mean velocity is often <1 m/s. By the time the Ganges reaches the plains, the gradient is only ~0.5-1 m/km, but the channel is so much larger and smoother (alluvium rather than boulders) that friction acts on a smaller proportion of the flow → mean velocity rises to ~1.5-2.0 m/s in normal conditions and up to ~3 m/s during the monsoon flood. The bigger, smoother channel more than compensates for the gentler slope.

Sediment size and shape. At the source, sediment is large angular boulders and cobbles delivered from valley-side weathering and rockfalls — sharp-edged because they have travelled a short distance. As particles travel downstream, ATTRITION (collisions between particles) chips off corners; sediment becomes progressively smaller and more rounded. By the middle course at Allahabad the bedload is medium gravel and coarse sand, moderately rounded. By the lower course in Bangladesh, the river carries primarily fine silt and clay in suspension (giving the water its characteristic brown colour) and dissolved load in solution. The Ganges-Brahmaputra delta in Bangladesh is built from this fine material, deposited where the river meets the slow-moving Bay of Bengal.

Discharge as the integrating variable. The Ganges' discharge at source is <5 m³/s. By Allahabad it is ~600 m³/s in normal flow; in the lower course it averages ~12,000 m³/s and peaks at ~70,000 m³/s during monsoon floods — a >10,000-fold increase from source to mouth. This rising discharge drives the channel-size and velocity increases described above; attrition drives the sediment change.

Examiner-pleasing synthesis. All three variables — cross-section, velocity, sediment — are LINKED. Rising discharge demands a bigger channel; the bigger, smoother channel allows higher velocity despite gentler gradient; attrition during the long journey reduces sediment size and increases roundness. The Ganges illustrates the BRADSHAW MODEL (1978) of coordinated downstream change, complicated only by the monsoon-driven seasonal pulse and by upstream deforestation in Nepal which has worsened sediment delivery.

The Bradshaw model (Bradshaw, 1978) is the textbook synthesis of how river characteristics change in coordinated ways from source to mouth. It identifies several variables that move TOGETHER along the course:

Variables that INCREASE downstream: discharge, channel width, channel depth, channel cross-section, mean velocity, sediment roundness, total load carried.

Variables that DECREASE downstream: channel gradient, sediment size, channel roughness.

The model is useful in three important ways.

First, it captures the COUNTER-INTUITIVE truth that VELOCITY RISES despite gentler gradient — because the bigger, smoother channel is more efficient (less friction per volume of water). Many students assume velocity falls; Bradshaw codifies the correct mechanism.

Second, it shows that the variables are CAUSALLY LINKED, not independent. Rising discharge demands a bigger channel → bigger channel is smoother → velocity rises. Attrition during transport explains the sediment trend. The river is an integrated, coordinated system.

Third, it provides a TESTABLE FRAMEWORK for fieldwork. Students sampling at multiple sites along a river can measure each variable and compare with the predicted trends. The model has been validated for many rivers worldwide.

However, the model has clear limitations.

1) It assumes uniform conditions along the course. Real rivers cross geological boundaries: a hard-rock band produces a knickpoint (sudden steepening, often a waterfall) that breaks the smooth long profile. The Niagara escarpment on the Niagara River and the Tugela Falls on the Tugela River are clear knickpoints that Bradshaw does not predict.

2) It ignores climate variability. A monsoon river (Ganges) has a strongly SEASONAL regime overlaid on the source-to-mouth trends. A snowmelt-fed river (Rhône, Indus) has its peak driven by TIMING of melt, not just by tributary inputs. Bradshaw describes mean conditions but not seasonality.

3) It does not account for human modification. Below a major dam, the Bradshaw signature is disrupted: the Nile below Aswan no longer shows the natural sediment-size trend because almost all sediment is trapped in Lake Nasser; the lower Colorado often fails to reach the sea because of upstream abstraction. The Bradshaw model assumes a natural river, but most large modern rivers are heavily managed.

4) It implies smooth, monotonic change. In reality, all variables vary site-to-site in noisy ways. Cross-section depends locally on bedrock type, channel-bed material and recent flood history. Velocity varies by position in the channel (faster in thalweg, slower at margins). Bradshaw shows the AVERAGE trend, not the local variability.

5) Some variables don't always follow the predicted direction. For example, deeply incised meandering rivers can be more sinuous downstream than upstream (counter to some expectations); braided rivers (Rakaia, New Zealand) have different channel-form trends.

6) The model is purely descriptive, not predictive. It does not specify HOW MUCH each variable changes, or at what rate. Two rivers with very different climates and geology will both fit the qualitative trends but with very different absolute values.

Judgement. Bradshaw is a USEFUL FRAMEWORK for understanding the BROAD QUALITATIVE PATTERN of river change. It captures the most-important counter-intuitive insight (velocity rises downstream) and the coordinated nature of the changes. But it is OVERSIMPLIFIED in that it assumes a uniform, natural, mean-flow river — ignoring geological discontinuities, seasonal climate variability, human modification, local noise and quantitative specifics. The best use of Bradshaw is as a TEACHING TOOL and a FIELDWORK BASELINE: useful for hypothesis-generation, but always to be tested against the specific river under study. In modern professional river management, more sophisticated process-based models (sediment transport equations, hydraulic models, climate-coupled forecasts) are used because Bradshaw alone is too simple. The statement is therefore broadly correct — useful as a starting point, oversimplified as a complete account.

Named river: the Ganges (India / Bangladesh).

The Ganges rises in the Gangotri Glacier in the Himalayas (~4,100 m). Here the channel is narrow and the cross profile is a steep V-shaped valley cut into hard, weathered rock. The river carries large, angular sediment from valley-side rockfalls and freeze-thaw weathering on its long profile of steep upper-course gradient. Vertical erosion dominates.

In the middle course (around Allahabad / Prayagraj), the Yamuna joins the Ganges, doubling its discharge. The channel widens to several hundred metres, the valley opens out, and the cross profile becomes much broader with a clear floodplain. Lateral erosion produces meanders. Sediment is now medium-sized gravel and sand, more rounded by attrition.

In the lower course (through Bangladesh, before splitting into the Padma/Meghna deltaic distributaries), the Ganges has a discharge of around 12,000 m³/s in flood and channels up to 1 km wide. The valley is essentially the floodplain — extremely wide and flat — and the cross profile is a low, broad U. Sediment is very fine silt and clay carried in suspension; the Sundarbans delta is built from this material deposited where the river meets the Bay of Bengal.

The trends are systematic: channel width and depth rise, valley profile flattens, sediment becomes smaller and more rounded — the classic source-to-mouth signature, complicated by the monsoon regime that drives the seasonal discharge cycle.