Once the positioning and style of your waterfall or stream has been decided, the amount of water needed to flow down the water-way will have to be calculated.If you would like the waterfall or stream used to return filtered water back to the pond, as is the case in many garden ponds, then the volume of water may be determined by the pond’s filtration flow-rate.If the volume of filtered water is too large for the water-way then some of the returning water may have to be diverted, this can be done by fitting a ‘T’ piece or ‘Y’ adaptor if possible, to the filter’s outlet, with one of the adaptors outlets set at the top of the water-course and the other been fed discreetly back to the pond. The separated flows can be adjusted with flow controls.

If the flow-rate from the filter is too small, an extra pump can be installed to add to the flow-rate down the stream or waterfall.It may be easier however, for your pond set-up to use a completely separate pump to power the waterway. As the filter should be left to run 24/7, this set-up would allow you to stop running the waterway while still keeping the filter running, should you wish.There are a few factors to take into consideration when working out the ideal flow-rate required for a waterfall or stream.

• Width of the waterfall or stream – Obviously a wider feature will require a heavier flow of water to fill it. The width of the waterway should be measured at its widest part.

• Desired boldness – The boldness of water describes the volume of water flowing down the waterway. A rapid, fast flowing waterway with plenty of agitation would be regarded as ‘bold’, a calmer film of water, moving at more of a trickle, is referred to as ‘thin’.
  

A range of waterfall widths have been tested producing the following table, which can be used as a guide to determine the flow-rate you require.

Width of Waterway

Thin Film of Water

Bold Film of Water

10cm

909 lph

1,818 lph

15cm

1,350 lph

2,728 lph

20cm

1,800 lph

3,673 lph

25cm

3,150 lph

5,455 lph

30cm

3,780 lph

6,546 lph

41cm

5,850 lph

9,547 lph

51cm

7,313 lph

11,933 lph

61cm

13,500 lph

18,184 lph

76cm

16,875 lph

22,730 lph

91cm

20,250 lph

27,276 lph

107cm

23,625 lph

31,822 lph

122cm

27,000 lph

36,368 lph

137cm

30,375 lph

40,914 lph

152cm

33,750 lph

45,460 lph

168cm

37,125 lph

50,006 lph

183cm

40,500 lph

54,522 lph

Once the ideal flow-rate has been determined you can calculate whether your filtration system’s turn over rate is suitable, or find a pump which will deliver that amount of water to the top of the water-way.Another basic, but usually sufficient method used for calculating a flow-rate for a narrower water-way is to simply allow 1000 Lph per 10cm of the water course’s width.