Complex sentence: Total water flow in the Murray-Darling basin in the period since 1885 has averaged around 24,000 gigalitres per year, although in most years only half of it reaches the sea and in dry years much less.
Split into simpler sentences: Total water flow in the Murray-Darling basin 1885 to the present has averaged around 24,000 gigalitres per year. In most years only half of this quantity reaches the sea and in dry years much less.