Thread by @rajbhagatt

Viz: #Kaveri river watershed 😍

I would be posting some #maps about the river with finer details in this thread in coming days! Bookmark..

FAQ
Data: SRTM
Software: QGIS

One of the biggest engineering works in Kaveri river is the Kallanai / Grand Anicut system. It was not constructed in 1 step & most of the structure that we see were erected during British era

Annotated drone image from @vikatan. The subsequent tweets deal with history of dam

Prior to the construction of the structures in Kaveri delta, it is likely that the river and its distributaries split in different places. The sediment deposition , erosion made the river to change the course constantly and through the years it developed many channels for flow

Somewhere during the sangam era, it is likely that the Kaveri river was filled with too much deposit & height of river bed rose. It led to the subsequent breaking of the banks causing the formation of Ullar river ((& Srirangam Island) and draining into Kollidam river

To prevent water going to Kollidam river & to retain water in Kaveri river (for Agriculture purposes), 1st step of engineering commenced leading to the building of Kallanai (Stone Dam) shown in #map. This was most probably executed by Karikalan but reconstructed many times over

In 1851 CE (British Era), two regulators were built to regulate the distribution of water between Kaveri & Vennar rivers. They were few kms above the Kallanai dam.

It was decided to bring the above regulators closer to Kallanai and in 1886 CE, the course of the Vennar was artificially changed. It was made to start closer to the Kallanai. The old course and the new course are shown in the #map

The new regulators for Kaveri and Vennar (course of which was modified) were constructed in the current site closer to the Kallanai and thus the "Grand Anicut" took a bigger and grander form!

It was not just grand anicut, in the 1800s the British (led by Sir Arthur Cotton & other engineers) built multiple other regulators, weirs, surplus canals that moderated and disributed the water flow between Kaveri river & the Kollidam. They were also constantly rebuilt & modifed

The final addition to Grand Anicut was Grand Anicut Canal. A dam was constructed in Mettur by 1934 & a new canal was constructed to irrigate more land under the Kaveri delta system which was also made to start at the same site

With the addition of GA Canal, the Kallanai or Grand Anicut took the modern form with multiple regulators that we see right now. Not all of them were constructed in same time!

The final leg (GA Canal shown in red colour) irrigated a lot of land that were earlier rainfed/tank/groundwater irrigated land. This newly added region is now called informally as New Delta. Mannargudi marks the border between the old and the new deltas

Even though nearly a century has passed since the construction of the GA Canal, underlying social differences still remain even in 2011. The Old delta has a higher proportion of Scheduled Caste population whereas the new delta has low proportion as shown in #map

Humpbacked Mahseer is a large #fish found only in #Kaveri & its tributaries. #Map shows where fish is found (area in red)

Dams & other human activities have pushed it to verge of extinction. New Dams(eg Mekedatu) would sound deathknell for fish
Data @stevenlockett @MahseerTrust

The primary source of #Bengaluru's water supply is #Kaveri river even though only a part of the city is in the watershed.
The city being in higher grounds than Kaveri pumps the water for a distance of about 100 km over a height of 1000 feet.

This visualization was created to show the elevation profile of the Kaveri pumping system for #Bengaluru City

Topography around Bengaluru can be seen in this visualization

https://twitter.com/rajbhagatt/status/1416036590539788297?s=20

#Kaveri is one of the most exploited river systems with thousands of manmade tanks (which many believe as natural) and other structures. Natural flow is limited to a few sections of the river and they correlate with habitats of native Mahseer

https://twitter.com/rajbhagatt/status/1455205225376018437?t=J-0s2BvXf8capxqxUiRAgA&s=19