By late May, the bed of the Vedavathi River in parts of central Karnataka lies bare.

People walk across it on foot. Cattle graze where water flowed just months earlier. In nearby weekly markets, traders speak of rain as something everyone is waiting for.

Then the southwest monsoon arrives.

Rainwater rushes down from the hills and, within days, the dry channel begins to flow again.

Farmers gather along its banks to judge how much water has come. Seed sellers report brisk business. Fishing nets, packed away through summer, return to use.

Across India's peninsular belt, this cycle repeats itself every year.

Some rivers are not meant to flow throughout the year. They return with the rains and fade when the monsoon leaves.

The rivers that follow rain

Geographers call them seasonal or ephemeral rivers.

Unlike glacier-fed rivers such as the Ganga, Beas and Ravi, which receive water from snowmelt and groundwater through the year, seasonal rivers depend almost entirely on rainfall.

That is why many rivers in peninsular India behave differently.

Take the Luni River, which rises in the Aravalli........

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