exodus 2:3 And when she could not longer hide him, she took for him an ark of bulrushes, and daubed it with slime and with pitch, and put the child therein; and she laid it in the flags by the river's brink. - Free Bible Online

      3. she took for him an ark of bulrushes--papyrus, a thick, strong, and tough reed.
      slime--the mud of the Nile, which, when hardened, is very tenacious.
      pitch--mineral tar. Boats of this description are seen daily floating on the surface of the river, with no other caulking than Nile mud (compare Isa 18:2), and they are perfectly watertight, unless the coating is forced off by stormy weather.
      flags--a general term for sea or river weed. The chest was not, as is often represented, committed to the bosom of the water but laid on the bank, where it would naturally appear to have been drifted by the current and arrested by the reedy thicket. The spot is traditionally said to be the Isle of Rodah, near Old Cairo.

JFB.