CREATE TABLE table_204_851 (id DECIMAL, "serial number" DECIMAL, "river" TEXT, "name" TEXT, "area  ( km2 ) " DECIMAL, "district" TEXT, "co-ordinate" TEXT, "altitude  ( m ) " DECIMAL)

what is the name of the river listed before chalakkudy ?
SELECT "river" FROM table_204_851 WHERE id = (SELECT id FROM table_204_851 WHERE "river" = 'chalakkudy') - 1
SELECT "river" FROM "table_204_851" WHERE "id" = (SELECT "id" FROM "table_204_851" WHERE "river" = 'chalakkudy') - 1