After the recent good rains of the past three months many are asking “How full are our dams now?”…and “Is Settlers’ Dam filling up?”. There is good news… and bad news.
After the recent good rains of the past three months many are asking “How full are our dams now?”…and “Is Settlers’ Dam filling up?”. There is good news… and bad news.
October produced the first good rains of this year with Howieson’s Poort recording a record (for 14 years) 205mm during October, followed by a moderate 76mm in November and an excellent 136mm during December.
From this abundance of wetness, one would naturally assume that the streams are flowing and the dams filling up. But what is important, is not always the total amount of rain, rather what sort of rain it was.
A 20mm downpour in 15 minutes from a thunderstorm creates far more run-off than a 30mm drizzle over two days…and it is run-off that fills dams. How many thunderstorms have we had so far this summer? I can vaguely remember one… and a few minor rolls of distant thunder the other day over the Fish River valley.
So our rain has been of the coastal frontal type characterised by fine drizzle spread over many hours, or days. Indeed we have experienced very little sunshine over the holiday period and I feel sorry for those who have travelled far from the interior to spend time on our formerly "sunny beaches".
The good news is that as of last Saturday, Howieson’s Poort Dam is full… and overflowing. This is the dam that is visible from the N2, to the left, just before the Kenton-On-Sea turnoff. It is immediately above and upstream of Settlers' Dam.
The main water pumps for Grahamstown are below the dam wall of Howieson’s Poort Dam, and the buried steel pipe runs up the Poort to the Waainek water works. I visited the dam on Monday and it is now a huge sheet of very clear water, with a good volume pouring over the spillway, and heading down into Settlers’ Dam.
The rapidity with which the upper dam has filled since being nearly empty in early October is testimony to the good state of its catchment. Howieson's Poort Dam receives water from the Berg River that rises along the Highlands road, the Palmiet River that flows down the poort adjacent to the N2, and the Oldenburgia stream that rises just above Rivendell Farm south of Featherstone Kloof and flows through the Oldenburgia Conservancy.
All these catchments have been partially cleared of alien vegetation over the past few years by both Working-for-Water and private land-owners, and the effects of this can now be clearly seen in the clear, fast-flowing streams now pouring water into our city water supply.
The value of mountain catchments such as that in the Howieson’s Poort area can not be over-emphasised as far as their ability to provide large volumes of good quality water – if they are preserved in a pristine state and not damaged by incorrect land-use. At present there is a flow of more than 100 litres per second of fresh, clean water coming out of the Rivendell valley, just two kilometres from the stream source, and more than three times that from the other two rivers, all pouring into Howieson’s Poort Dam.
That’s the good news. The bad news is that Settlers’ Dam is still nearly empty. It has risen by about a metre, thanks to the water coming from Howieson’s Poort Dam since last Saturday, but virtually nothing is coming in from the Kariega River, the main "arm" of the dam on the southern side.
At the picnic site, there is still no water and the dam only just extends past the yacht-club slipway by about 50 metres at present. The dam on Mosslands, above Settlers’ Dam, straddling the N2 on the Kariega River, is still very low, testimony to the poor flow of the Kariega River to date.
Why has this river not flowed, despite the good rains? A good question, and one that poses some interesting scientific questions as to why some streams are flowing steadily in the district, while others are still virtually dry.
Along the Highlands road, both Jameson and Milner dams are still very low, although Jameson has risen a little. Both are at less than 10% capacity. If we are lucky enough to have some heavy thunderstorms over the next few weeks, with the current wet state of the soil in the catchment, there should be good run-off, and we would then expect a substantial inflow to these dams.
