MR. DOUSLIN AND THE HUTT RIVER.

Marlborough Express, Volume XIV, Issue 1122, 5 April 1879, Page 7

MR. DOUSLIN AND THE HUTT RIVER.

(Continued from our last Issue.) The first darn .1 erected was on the south hank of tiro Opawa at .Dodson's bend, which had been caving in for Jive or six years. When the dam w„s fixed there was 12ft of water. The bend where the dam was creeled is now dry, and the river forced 200 yards into the opposite hank. After the undoubted success of this dam, another was constructed at Leary’s, and one at Hock Ferry. 1 have sent you the printed reports respecting Leary’s breach. These two places have been for years the most dangerous to the district, and especially Rock Ferry. This was the site of the main branch out of thcWairau, and which branch constituted the Opawa River. On the north side is a large rock which stands above the water 40ft.

About a quarter of a mile above this we come to the foot of the main ranges, which are very bluif, and the river is close under the bills in this vicinity. The river lias boro excavated the rock for its bod. This is also the confluence of the Waihopai River. Then, between this point and the rock, the river has excavated an arc of about a quarter of a circle. This sinuosity and the lower rock have, and will always play a most important part with the condition of the river by deflecting the main current towards the plain and town. Sometimes this bend is filled in with gravel, and runs almost in a straight line. The axis of the current does not then strike the lower rock. (I want you particularly to understand this position). This was the condition of the river when J first fixed the floating spur, but in November, 1877, a very high flood—the highest that was ever known in the Wairau—swept out the bed of the river round the bend, and a convex gravel bed formed on the opposite side of the river. You will see the operating cause in directing the river had entirely changed its physical character, and the location for the floating spur was selected under the former condition of the river. The floating spur was completed in August, 1876 ; it is 250 ft long, and cost -(,‘280. The month of the Opawa Paver was just below the dam, and two-thirds of tiro volume of water was going down. It was the largest breach over known in the Wairau IHver. About 2 miles below this breach there was another at the head of Pox’s Island, which led into the Opawa. The sketch in the margin will give you some idea of the condition of the river, and floods up to this time were of frequent occurrence. have been able to pull a boat across Market Place, two dr tlireoTiunos'wifliih Since the spur was erected the centre of Market Place has never been hovered wiflr. waleix "'Well, Pl'iiukt n'aWlMricTrio the breach. From August 1870, to February 4, 1877, we had a few medium floods. 1 carefully watched the river after every flood, and I had the satisfaction of discovering thrfc the breach in the Opawa was getting less and less, until the 4th February, when we had an unusually high flood. The storm was of short duration, hut heavy, and soon came down those mighty mountain torrents charged with the spoils of the hills and valleys, and this breach was closed, and when the river subsided there was uo trace of a breach, and it was as if no river had been there. The geographical features of the place have changed. Gravel was piled up across the mouth of the Opawa, 4ft above the level of the water, find extending back for chains, and the mechanical power engaged in piling up this mass of material across the breach was that of the Wairau Paver, and the river is the only power that can be brought to bear to do any permanent good. This sudden cnaugc concentrated the whole energy of the river to one channel, to the head of Foxe’s Island, the locality of the second breach. The result was the deepening of the bed, and also continuing to deepen its bed below the island, drawing off the water from this second breach, causing an increased velocity in the "Wairau, and decreasing the velocity in the Opawa, which left the river powerless to carry forward its shingle, and the consequence is that this breach is fast filling up with shingle—it is now a long shallow bar. This change just reversed the condition of the two rivers in this way. Previous to the erection of the dam the Opawa was continually drawing water from the Wairau, which increased its velocity and deepened its bed, while it left the Wairau River powerless, and it had to lay down its sediment, forming shoals and bars. Since the breach was closed the condition of the river has been reversed, and every flood wall increase the volume of water down the Wairau River. In November, 1877, we had a very high flood in the Wairau, and it scoured out the bed on the south side of the river between the rock and the foot of the range (previously referred to) ; You will readily understand that the centre <jf effect was at a widely different angle to what it was when the spur was first fixed. The operating cause had changed, and the centre of effect was directed to the north side of the river immediately below the dam, and dividing the Wairau into two branches, the southern breach returning again into the main branch of the Wairau river, above Foxe’s Island, ahd about the second breach. I prepared plans for lengthening the dam, but they were not carried out. Below' the rock the river runs through alluvial ground out to the sea. For three or four miles below the rock, the banks are composed of shingle which has been the river bed in days gone by. The vicinity of these rocks (at Rock Ferry) has, and alw r ays wall, operate in direeting’the centre of effect. If such is the ease, it is clear that some obstruction is necessary on . the south side of the river to receive the impact at the angle to which the current is to he deflected. Had the dam (on the south side) been extended 150 feet, as was recommended, the south in the Wairau could not have occurred, and if it was extended 150 feet, it would close np. The people interested were quick at discovering the effect, but slow to find the cause. Whatever works are undertaken for the conservation of the river, this strong position must first he attacked. £13,000 had been expended in various parts of the river, before I undertook the matter—two or tliree miles below the rock, and there is not even

| a memorial left of C' ■ year by year the Opa\w» m na.i'ed in volume, a»4 devastated large tracts of valuable land. The lloating dam was the first and only work ever erected near the rock. No one thought of attacking the enemy in his stronghold, and ever since it was erected, the Wairau river has increased, and the Opawa decreased in volume. When the Wairau river commenced to branch, it lost its carrying power, and tire result of this lias been to leave deposits on its course, and raising its bed. Matter that passes on in suspension under the impulse of a current of five miles an hour, null be precipitated in currents of three or four miles an hour. And some portions of the river bed have filled up many feet, and there is a general rise in the riverbed. If the whole stream was condensed into one volume, it would increase its velocity and give it a carrying power that would transport its silt out to sea, while vdth an inferior carrying power it would sink. Of late years the floods attain their maximum height in a much less space of time than formerly; this arises from various causes, and one prominent cause is the destruction of timber in the valley, and setting fire to the sides of the hits and destroying the herbage, every plant of which assists to retard the rain water which now finds its way rapidly to the rivulets. The burning of the hill sides is followed by tire soil slipping off to the bare rock, and carving great chasms in the clay and destroying the face of the slopes. Another cause is the clearing of flax and scrub, and the draining and cultivation on the low lands. In several places near the hanks of the river, especially on the Opawa, very substantial embankments have been erected, which have been of great service to the district. There are certain local specialities which exist in most rivers, and the location of the floating dam is of vital importance. They should he fixed above the erosion or breach, and extended some distance beyond the current’s centre of effect. Should the Council or Conservators desire to fix one of the dams, I shall feel richer by an acknowledgment of its success than, any money payment could make. I am, Ac., — Wm. Douslin.