Here is a 10-point lowdown on the water sharing dispute:
About 76 km of Mahadayi's 111-km length runs through Goa and the rest is in Karnataka. It passes through Maharashtra briefly and meets the Arabian Sea in Panaji, where it is also called Mandovi.
The dispute over the sharing of river waters started in the 90s when Karnataka designed a chain of dams and canals under the Kalasa-Banduri Nala project to channel Mahadayi's water to the parched Malaprabha basin in north Karnataka.
Karnataka pegged its demand for Mahadayi water at 7.56 thousand million cubic feet to meet the "drinking requirement" of farmers in northern districts, but Goa, which had raised objections to the Kalasa-Banduri project, said it is water-deficient and sharing any water with its neighbour would cause ecological damage to the state. The clearance for the project was then put on hold.
In 2002, Goa demanded that a tribunal be set up to assess the situation. After several attempts at negotiation failed, the Mahadayi Water Disputes Tribunal was finally set up in 2010.
Karnataka sought the tribunal's green signal for the Kalasa-Banduri project, arguing that the river must be used for drinking purposes instead of letting it out into the sea. But in 2016, the tribunal rejected the demand triggering protests in the state.
With elections due in Congress-ruled Karnataka, political parties will go all out to woo voters and the Mahadayi issue seems to be among every party's preferred choices.
Last month, state BJP chief BS Yeddyurappa got in touch with Goa Chief Minister Manohar Parrikar asking him to consider Karnataka's drinking water needs. Mr Parrikar wrote back the next day saying Goa was ready to share a "reasonable" amount of water "on humanitarian grounds".
The exchange of letters, however, sparked outrage in both states. While Karnataka Chief Minister Siddaramaiah took offence to Mr Parrikar responding to Mr Yeddyurappa and not the chief minister of the state, Goa Water Resources Minister Vinod Palienkar declared that the state will not share even "a drop of water" with Karnataka. He even called Mr Parrikar's gesture a "political stunt".
Afterwards, there were protests by farmer groups outside the BJP office in Bengaluru criticising Mr Yeddyurappa for failing to deliver on his promise. The BJP, on the other hand, accused the Congress of sponsoring the protests.
Miffed farmer groups now blame both the Congress and BJP of playing politics at their expense. They approached the Karnataka Chief Electoral Officer a few months ago to urge him not to hold assembly polls in the state until the Mahadayi dispute was resolved.