CANTERBURY RIVERBEDS AND BEACHES.

Press, Volume XLVIII, Issue 14432, 13 August 1912, Page 5

CANTERBURY RIVERBEDS AND BEACHES.

SOME OIJSEKVATIONS'AND .THEORIES.. (By H. E. Mpsgbavb.) (whitten fob "the press.") , As there seems to bo a good deal of ignorance as to what is being done along the coast of Canterbury by natural forces, I will endeavour to explain, to the best of my ability, what I have observed during twenty-nine years' residence in Canterbury. To understand our shingle und sand beaches, we must first study how the material of which they are composed rtaches the coast, and to do this we must go back to the main ranges of the Southern Alps. The denudation of the mountains by the action of the elements, ass.sted by frosts, provides a continuous supply- of shingle, which is wrought down by the rivers and doposited on the beaches. There is no doubt that - a greater quantity of shinyle has been brought down tiie rivers since tiie country uas been settled than before settlement, when tno rank growth of grass on tiie iUiis, and tiie, at one time, large acreage of busii, allowed the rainfall to firadua.iy iiiter down, into the various watercourses and r-vers, so tiiat they did not tiood rapidly, tiiue'e tiie tussocks and a good deal of the bush have been burnt, and tue remainder tut down, the water comes oif tiie hills very rapidly, causing tho rivers to flood quickly and carry a larger quantity of shing.e down thor courses, so that, to my m;nd, the beaches nave increased more rapidly wicii.n the last titty or sixty years than they did before. Tais rapid transit of tno water from the liihs to the sea is the cause of so many Canterbury riverbeds being so mucii wider tisau tiie average stream of water that they carry. Obstructions, such as gorse and wiilow, constantly turn the river from one side to tiie otuur, and carry away the banks. Aiiocher c-itect of ti.ose obstruct.ons is that the beds of all our rivers are in tuuir lower reaches, and in some cases are Setting above the ievel of the surrounding land, wh.ch land is some of the best in tiie Dominion. 'ihi» applies more especially to wi:at I may catl the intermittent rivers—tJaoso which do not ran aJI the year round —whose waters disappear ut a gent-ral average of about ten miles from the coast, but &till continue to flow under the shingle and reappear at the mouth. Such nvers aro the Pareora, Orari, Hinds, Seiwyn, Kowai, nnd Waipara. One nrotmnent instance came under my notice recently. About fourteen yoars an'o a bridge was. built over the Ornri on the Main South road, just below the railway bridge, ten miles from the- coast. The riverbed at that time was twelve or fourteen feet below tho br.dge deck. I crossed the river there last >Anril, and was astonished at the rapid rate at which the riverbed had risen, it was nearly up to the cap of the piles. The bridge has had to be raked about six- feet, and was just finished when, I crossed. This shows that a largo part of the sbing!e in tuese interm.ttcnt rivers does not reach the coast, but is piling up in the riverbeds, and this means that something will have to be done, and that soon. My friend the late Mr W. U. Slack, wrote to the papers in South Canterbury twenty-five years ago stating tbnt eventually the riverbeds would have to be dredged; overjbody then, including

•.ayseif, iaughed at the idea, but ho .as r.gbt. "Witb this digression wo will now go back to the beach. The jshingto. having now been brought down to liie beach, is met by a svro;,g ocean current running from south to north, which is continnouslj moving it along in a northerly direction. 'J -us ocean current is the southern equatorial current, which sweeps from the oquatur in a south-westerly direction <lottii tin ,east coast of Australia, thence southeasterly towards New Zealand, then due oast, running up both coasts of New Zealand away, to South ; then north and aguin west along the equator, its course forming a circle. As evidence of this northern movement of the shingle, you have only to look at Banks Peninsula, viiieh stands out at right angles to the coastline. All the shingle from the Waitaki, the Opihi, the Rangitata, the Rakaia, and tho other intermittent rivers before mentioned, which has been travelling north for hundreds of years, is trapped by the Penir.sula. Lake EUcEmere was no doubt at one time open sea, and nil the valleys from Tai Tapu to Little River were bays. Gradually the shingle crept along, accumulated, and formed the boulder bank which now renches from Tanmutu to Ohakoa. Lakes Ellesmero and Forsyth being orly the belj wvn and Pohoa rivers blocked up by shingle. The Timnru harbour works are another instance. When the break- ' water was firot built, Canoino Bay ! had only a small strip of sand, and the sea came right up to, and sometimes over, the .North ioad. A s> tho siiingle gradually collected oelimo tho . breuiiWuLvir, only tno sand was carried past, and the Lay began to lili .up rupiuly. When the travelling I shingle got past the end of the breakwater, aud Timaiu foui.d that its harbour entrance could not be kept clear, even by dredging, another rubble wall was built, and uio process of trapping ' shingle and reclaiming land f.-'gan again. If Tiniaru intends to retain its harbour it will have to go on extending its breakwater indefinitely. It was observed that tho beach north of limaru a= far as tho Orari" got much more sandy after the breakwater was built. Akaroa Harbour is- experiencing the > same thing, as tho fine sand which gets ' past tho end of the boulder bank at '■■ Lake Forsyth is gradually silting up German, Duvauchellcs, and Barry s Bays. . I in Pegasus Bay, the south equatorial I current is having the same .fleet as jM ' South Canterbury, only in the cout.su y direction. ,■■*>■• I When the current leaves Banks Pcnin- ' sula on its way north, it strikes the coast again in the vicinity of M 'toiiau Island, and creates an eddy or bacKwater whirh becomes a current running south to Whitewash Head, thence south-easterly round tho bay to tho main south current off Long Look-Otit. Between Motonau Island and Whitewash Head five rivers empty their waters, charged with shingle and , sand, into Pegasus Bay. Now three of these rivers, tho Waipara, Kcwai, and Ashley, bring down vc/y soft, friable shingle, which soon grinds up into sand. Of tho oilier two, tho Waimakariri brings down practically no shingle beyo.d ■ White's Bndgo, and the Avon only sand, j which is deposited on tiie beach. , Tho I prevailing current being, as .ye have ! seen, fr-m north to south, all*the samt i deposits from theso five havo lor ages been brought from the North and placed in an ever widening belt iroiu Leithfield to Christchurch. The beach ' from the cliffs at Teviotdalo to tho . Kowai is composed of small shingle and ■ sand. There are patches of him sand, i which you can drive over, r.nd tlien ; you suddenly break through into soft I shingle. South of tho Kowai the sand boach and tho sand dunes begin, .J. ho ' latter extends gradually further inland till Christchurch is reached. That the high-water mark is receding is very evident. I have heard of a ppsfc at Woodend that twenty or thirty, vears'ngo stocd at 1 - high-water mark, but wh.cb.--is now the best part of half a milo inland among -the sand dunes. At New Brighton it has been noticed for some years that the beach is making fast along tho Esnlanado,' more-rapidly sinco tho marram grass planting at high-water mark was begun, in order to thus hold and bind the sand as it .comes' ashoro and prevent the preva ling east wind from carrying it inland, if tae sand was not held on the foreshore it would keep drifting in amongst the houses and" down the streets, as it usea to do a few years ago. The garden riots north and south of the pier r.re now only kept clear of sand by continually scooping it away during the summer months. Eventually marram grass will have to ho planted in front of them, or else the sand will overwhelm them and go blowing down Sea View road as of old. Any intelligent observer can see that the..beach is gradually rising at the pier in spite of tho continual swooning. That tho leach is r.sing more rapidly since the marram grass planting boKan was shown by what happened «v the recent spring-tides. Last year they washed out a good deal of newlyplanted grass from south of the pier to Bowhill road, and got in amo.-g tho sand hills to tho north. This year the highest tide, with a heavy easterly sea behind ;t, only came up to tho edge of tho marram grass, but north of iiowhill road, where there is no marram crass* it went many chains further inland than it did last year, as anyone may sco for theirsclves by observing the d:-ift-wtod away in among the sandhills. It will be seen from this study of sand accumuiat on, and receding high water, mark how utterly impossible it will be for New Brighton ever to have a sea-wall. An esplanade may be I laid out in time on ton of the sanddunes, but U> will have" another sanddune on the east side of it, between it and the sea. Br ghton must go on petting gradually further away from the soa.. It" cannot help it. No one can alter permanently, the forces of i\ature hero represented by the south equatorial current, the Pacific ocean and tlw east wind. In fif ty t years time, some people think less, thA high-water mark will be abreast of the seaward end of- the r ]er and the sand-dunes up to tho level of ilio pier deck. I have observed that a urcat deal more marram grass planting has been done from the Kowai to the Ashley than , has been done further south. Hundreds of acres have be n covered, and any bare places occurnag aro at once replanted. Cattle and sheen run all over th's dune cocnt-y and seem to thrive. The fact that all the millions of tons cf shingle brought down by the Waimakariri do not cet past White's bridge is a somewhat serious proposition for Christchurch, and :t rr.ny acconnt for the very serious flooding about Kaiapoi and Flaxton, in or during the last few years. Timaru will have to go en studying travelling shingle, and Canterbury generally must soon urgently cons der her rising rivcr-bsds. If the map of Canterbury is studied, it will bo seen that thero are many rivers and streams between tho Pareora in the south and tho Mason in Amuri. I have a fair i acquaintance with them all. I have crossed them by ford and br'dze at many places on their course from the hills to the sea. I have met them in flood and in normal condition. On ono occasion, only a little over two years ago, I was held up in the Mason riverbed for a week, so it is nossible to net od-timo experiences even" in these days of motor-cars and their necessary accompaniment, the ever increasing ?. c " h lived for seventeen years in tho Goraldino district, and for the past p • v* Years on the Psplanndo at New H"! • • an d claim a fair amount of knoTT-Mee of b-th river nnd beach. *ew Brighton, August 12th.

■ Two guineas for four lines of poetry. Ivead Ponking's Linseed Emukkn intimation every Saturday amongst aews items. 3