What crops are grown in the nile valley?

The 'Nilometer' which was really a rock marked vertically at equal spaces, could tell the farmers by just one look whether they could afford paying off much of their crops as tax to the King. It worked because the Nile flooded numerous times and always left plants at the height to which it rose whilst flooding. But this created problems too. Try to think about what those problems could be. Better still, get it on to the net so everyone can see.