CHAPTER 3 GEOMORPHOLOGY the Vast Expanse of the Landscape
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?8 CHAPTER 3 GEOMORPHOLOGY The vast expanse of the landscape hidden behind the semi-transparent screen of mist and dust, dotted with crenulated hills soaring high in the sky with deep winding valleys and the interfingering gullies frantically competing for space, poses an attractive but a complex problem to a student of geomorpholo^y. The landscape is a natural product of the various interacting processes. Climate is one of the important variables that influences the operation of geo- morphic processes. Regarding the factor of time, nothing can be said with certainty as to the exact period over which these processes operated to produce the present topography. The incision and dissection of the plateau, by the Krishna river system has resulted in evolving the present topography, which is dominantly formed of valley slopes. The all embracing scenery of the region, constituted of mountains 29 and valleys, rivers and waterfalls, cliffs and canyons, flood plains and talus cones, is the product of the fluvial processes. The region is covered by residual hills and valleys interspersed with planation surfaces. Field evide nces indicate that the landscape representing more than one erosion cycle under the influence of humid and semi-arid climate is, polycyclic in nature. The geomorphological studies carried out in the area encompass various methods helpful in qualitative and quantitative analyses of the landforras. These studies are aimed at evaluating the neotectonics and climatic changes in the area and are used in discussing the genesis of late- rite in Chapter 4. This is attempted by' a) Studying the evolution of the drainage system b) Identifying and interpreting the planation surfaces; and c) Studying the development and influence of slopes. This chapter includes the studies pertaining to longitudinal profiles, fluvial morphometry, areal and altimetric analyses, drainage lineament studies, and slope analysis, which are described'in the following pages. An example of the river capture in the area is also described in the present chapter. THE RIVERS The region is drained by the trunk stream Krishna and its tributaries. The right bank tributaries rising in ^(1 Western Ghats, include Kudali, Venna, Urmodi, Tarali, Mand and Koyna of which Koyna is the important feeder. The left bank tributaries rising in Wahadeo hills to the north of the area are Vasana and Yerala of which the latter forms an important feeder. KRISHNA It rises on "/estern Ghats at altitude around 1350 metres near Mahabaleshwar. It flows almost across the entire breadth of the Peninsula from west to east and dis charges its waters into the Bay of Bengal. Out of its total length of 1480 kms, and its total drainage area of 270,500 sq kms, the part of its upper reaches selected for the present studies include a length of 260 kms and an area of 10,450 sq kms. The river flows eastwards for about 25 kms from the source and then receives Kudali near Udatare> from where it continues its coxxch-^ in southward direction up to Limb. It is joined by Venna river further downstream, at Sangam Mahuli, Urmodi river at Kashil, Tarali and Mand rivers at Umbraj and Koyna at Karad. The confluence of the Krishna and Yerala rivers at Digras is at a distance of about 260 kms from the source. The general direction of flow is south easterly. SINUOSITY OF KRISHNA The overall sinuosity of the Krishna was determined by the ratio of the channel length to valley length (Leopold ,1t et al.,19S0). It v/as calculated to be 1.37. On this basis the present day channel of the river can be classified as straight or sinuous channel (Leopold et al., 19^9) or as single channel v;ith lev; sinuosity (Rust 1978), because the sinuosity value is less than 1.5. Further, the sinuosity values in 16 segments of 10 kms valley length, dowistream of Dhora was determined. The values fall between the limits 1.02 to 1.80. This indicates that, in certain stretches, the present day channel is highly sinuous and meandering. Rust (1978) proposes a measure for braiding intensity, as the braiding param.eter. It is expressed as the number of braids per mean meander wavelength. Accordingly, the alluvial systems are classified as single channel system having braiding parameter equal to 0.5, multichannel mode rately braided system, with parameter equal to 2.0 and multichannel highly braided system with parameter as 6.0 , The present day river channel of the Krishna shows braiding only at a fev; places as at Taswade, Umbraj, Bahe and Borgaon and Burli. The braiding parameters calculated at these locations give the mean values less than 0.5 indicating the alluvial system to be, single channel alluvial system. VENNA It is one of the chief feeders of Krishna and originates at an altitude of about 1250 metres on the Maha- baleshwar Plateau. It pluiiises down into the valley below Lingmala forming Lingmala falls (Plate 3A). Here, it leaps over a vertical cliff of basalts,about 30 metres high. It flows further nortnwards for a few kms and joins Krishna at Sangam Mahuli near Satara. For most of its course the river bed is rocky and covered with coarse debris containing pebbles and cobbles that exhibit a high degree of rounding. Downstream of Kelghar, there are knick points and many rapids along its course up to Medha. URl^ODI It rises at an altitude of about 1100 metres. It flows in the south easterly direction and joins Krishna near Kashil. Its valley is broader than that of the Venna. Near Valse the bedrock is being cut so vigorously that the pot holes nearly 2.5 metres deep are drilled in it. KOYNA It rises near the Elphinstone point in the western part of the Mahabaleshwar plateau. Of its course of 15^ kms, it has a N-S course in the first 55 kms downstream of its source. At Helwak, it takes an abrupt eastward turn. From this point it moves in the eastern direction up to its confluence at Karad. Within this distance, Koyna is joined by a number of tributaries as Solshi, Kandati, Kera, Morna and Wang. In its north-south course, the valley floor is seldom broader than 30-35 metres., but in its easterly course, it widens up to 150 mtrs. The valley sides show steep slopes and bear a dense vegetation. YERALA It is the largest of the left bank tributaries in the area and rises at an elevation of around 950 mtrs. It has a nearly north-south course for about 110 kms from the source. Further downstream it flows south-westerly for about 40 kms and joins I^ishna at a place 2 kms upstream of Digras. It has low banks and flov/s through semi-arid zone and has Chand R. and NaniR., as its tributaries. LONGITUDINAL PROFILES Longitudinal profiles of the Krishna river and its five important tributaries Venna, Urmodi, Koyna, Vasana and Yerala were constructed ajid com.pared (Fig. 3.1). The profiles were obtained by plotting channel heights against the dista nce from source measured along mean course of the valley. The topographic maps of the scale 1'63360 were used for the purpose. The profiles indicate that the ICrishna and Koyna rivers have very high gradients, equal to 104 m/kra and 110 m/km respectively, in the source region (.Table 3«l). Gra dients lower down further, to 5.2 m/km and 2.8 m/km respe ctively, in their confluence zone. It is also observed that in those parts of the valley, where the valley floor is above the average level, the river is confined to a narrow channel. In the intervening reaches the channels lie in gravels and are bordered by varying widths of alluvial c rvi o — Q. o 1—2 t - tn C/1 o f— < t—1 cc < o Q > u z < < QC z: ^ - Q X in E Q. (/I t:^ LU ^ O CL CO UJ Q: < < -z. I— Q CD 3 O u. s3ai3H aaniinv Q^ 05 <U O LPi H 05 •H CO O H CO •H TK CO o o -P •H CO in• CO• in w CM o CO hJ > s o u -a •H (J^ (U o 4-> o CO in C« a CT. O H CO O to CO O 00 CO o u (X) > s o H •H a: to o in CO CO • • • u o \£) CO (U o S 1^ •rH CO -p CO C! CO OJ o •H CO in •H o CO QJ CO 1-1 >-t 1J o -4 dN 0) •H o H fc>0 CO OJ CU Q) o CO !U f^ EH 0) 0) CU :3 o H rH IH TJ ^ :3 XJ G o •H o CO S o •A depositsj so that, the rivers have the appearance of being fully graded. The profiles of the Krishna and its right bank tri butaries are similar in form. All of them have their profiles concave upwards and very much regular and smooth. These profiles appear to be of the rivers approaching near- grade conditions. Profiles of the right bank tributaries differ in thedr form slightly from the left bank tributaries- Vasana and Yerala. Their profiles are not perfectly concave upwards and appear to be slightly convex in their middle and lower reaches. It can thus, be generally said that the slopes of almost all tributary valleys are similar in form to that of the main valley suggesting that the river system has reached maturity (Jones 1924). The comparison of the profiles of the Krishna and Koyna, indicates that the Koyna valley after a few kilometres downstream of its source, attains a lower level than the main valley, emphasizing a higher rate of downward erosion.