Everything you ought to know about Graded River Profile

Now we take up the complicated question of the grading of long-profile. So long as a river flows from source to mouth at every point on the course, it tends to grade the channel. In the process of flow, the stream might be eroding the discordances or rocky projections in its course or filling up the depressions. All this will be related to the geology, slope, water discharge, the size of sediments, the quantity of sediment load, the velocity of flow, etc.

The term grade, however, for a long-profile refers to the whole course and is a general description suggesting that the profile from source to the mouth is as a completely generally smooth and the irregularities and discordance have been evened out. No doubt, this grading is with reference to the base level which is the sea or the confluence of a tributary with the main river. As such the idea of grade has been described as indispensably valuable by Woolridge and Morgan.

The graded river curve is a concave profile. A graded stream is defined as one, which is neither eroding nor depositing. This cannot be true for any stream if the whole course were considered. This condition can hold only for short stretches in the middle section of the course. As for example, the Ganga is eroding in the Himalayan section. It sheds a good deal of its load when it leaves the hills. In its course in the plains, it carries its own load as well as that of its tributaries. This may be spread in the surrounding plains during flood over spill. The over-spill is a function of flood-causing factors like short-duration storm rain and the river profile may be temporarily irrelevant. Thus flooding will be associated with deposition. When we talk of grade, we cannot confine our attention merely to the channel of the river. The channel has at any rate to be kept clear for the stream to flow. Therefore, the process of grade must involve the areas ‘along’ or adjoining the course of the stream.

In the case of the Ganga, there is down cutting by the river in the Himalaya. In the upper part of the plain section, the banks are free from levees, the channel relatively straight and deeper than in the middle and lower parts of the course, e.g., eastern U.P., Bihar and West Bengal. In these latter areas, particularly Bihar and West Bengal levees are prominent indicating marked deposition along the course. The sediment load is so high and deposition so marked that the stream is filled and divided into numerous distributaries, which are associated with, levees. Seaward deposition causes outbuilding of the delta. Thus, the lower section of the Ganga is in a stage of aggradations. The upper Himalayan section is one of downcutting or ‘degradation’. The upper middle section alone may be regarded as one where no marked deposition or erosion is taking place and the term grade may be applied only to this section.

The conventional definition, which says, “a stream is said to be at grade when it is neither eroding nor depositing material at ay point along its course” will probably not be fulfilled by any stream. It is probably an ideal, which is never realized. There may be seasonal deposition and erosion in the same part of the channel. When the volume of the Ganga or any river of a plain falls the sediment deposited where the discharge and load was large during the monsoon is exposed as sand banks or islands. This may be subsequently eroded if the current of the stream impinges on it. Such deposition of large mass of sand can be seen in the bed of hilly streams of Chotanagpur also. It may subsequently be eroded when the discharge rises after a shower. Thus, deposition and erosion are non-ending processes along a stream. The curve of long-profile as a whole may be designated as ‘graded’, but in the sense that this profile is generally smooth and free from discordances, not that the stream is neither eroding nor depositing at any point along its course. Such a graded long-profile is on the whole concave.

The development of such a graded profile, provided the rock is homogeneous, will be achieved when one-fourth of the rock mass of the basin has been removed by erosion.

Grading of streams may be described as a tendency of a river to change or adjust its shape and slope, under the given conditions of discharge, velocity and size of sediments in load, “so that the load entering equals the load being passed on.”

Three types of profiles of equilibriums have been shown by Fair-bridge. One is the ideal theoretical profile of concave shape. This is achieved in short rivers. The second profile is stepped type or undulating. This is the characteristic of rivers with tributaries in basins of homogeneous rocks. After every confluence the discharge is increased and also its competence, i.e., the capacity to carry larger load. The greater erosive capacity resulting from increased discharge steepens the gradient immediately below the confluence. The third type of profile is stepped or Tenckian’ or treppen type. Here the basin is believed to be a series of pediments with a knick at the junction of each successive pediment. The profile of the larger Chotanagpur streams is of this third type. The Subarnarekha is a representative of this type. As the river leaves the older and higher erosion surface, we have the spectacular Hundru falls.

If this profile is not accepted as Penckian treppen type, it is definitely one running across different levels, which might have been occasioned by marked faulting and uplift. In all probability, the profile on the pre-tertiary peneplain or erosion surface of Chotanagpur was a gentle concave profile. The discordances in the long-profile of streams now seen are due to retreat of the margins of the various erosion levels the junction being marked by the discordances. So here, the discordance is due to parallel retreat of knick-points or due to faulting and relative dislocation. The streams of the peninsular upland that descend by discordances on the scarps of the Western Ghats are due to tectonic dislocation in late Tertiary.

Now homogeneous structure, which has been assumed as a pre-condition for the undulating long-profile, shown by Fairbridge, is not extensive over large areas. Variety rather than homogeneity characterizes most of the lithological formations. There are ‘geomorphic’ (i.e., resistance to erosion) or otherwise variations within the same structure, e.g., the granites or gneisses or schist, etc. of Peninsular India. The extensive homogeneous formation, e.g., the Deccan lava too has lithological variations. No doubt, the Indo-Gangetic plain has a remarkably homogeneous lithology and here the profile of streams is very smooth and uniform.

For the development of the characteristics of different types in profiles, sufficient relief is essential. The true profile types illustrated by Fairbridge need to be carefully reviewed. The first smooth concave type cannot develop unless there is considerable relief in the source region and if the headstream region has considerable relief it must be hilly or mountainous. If this were so, tributaries are so frequent in hilly terrain because of dissected or irregular terrain that a river without tributaries will not be a reality except in the case of puny first-order channels and as such, they do not merit much attention. However, it is a fact that smooth and concave profiles do occur in the case of larger streams.