What details lead Ishi to believe that Elder Uncle and Tushi have not survived the escape?

Ishi is worried that Elder Uncle and Tushi have not survived the escape because the rain has caused the river to overflow and become dangerous and muddy. Elder Uncle forgot his walking sticks, so this makes their traveling even more difficult. In the mud along the banks of the river, Ishi finds the necklace that he made for Tushi, which is a sure sign that something has gone wrong with their escape plan.