IN TOUCH WITH NATURE

Hokitika Guardian, 22 March 1930, Page 7

IN TOUCH WITH NATURE

WATM’IiFAI.I.S AND LIVERS

(Hy .1. Drummond, F.L.S., F.Z.S.) Some oil’ the most beautiful waterfalls are seldom seen by people. They often sing their melodies and raise their voices loudly in places that are almost inaccessible. Birds and insects may he their only audiences. Amongst these little-known waterfalls are two very beautiful ones near the head of the Waitaha Valley, South Westland. With its .source in a very smaller gl ne'er tucked away in a nook in the Southern Alps, the Waitaha Hirer has its headquarters in a deep valley on the side of the Lange Lange. Two large streams, coming down the waterfalls and cascades from cliff-glaciers, join the Waitaha and swell its waters. \Y,w a fairly large river, the Waitaha (lows for more than a mile through a ['-•shaped valley, or hanging valley, at the end of which it tumbles!' for 80 feet in a waterfall. Further drown, the river, steeply descending, falls 8800 feet in three-quarters of a mile. After it is joined hy the Chninmnn Stream, it plunges into an inaccessible gorge, with precioices from 400 to 800 feet hiprli. The Waitahn’.s most notable waterfall is about a mile and a half from th-3 head of the river. The water drops between 80 feet and 100 feet.

Describing these conspicuous and fascinating features of this very picturesque South Westland river, the late Mr P. 0. Morgan, director of the New Zealand Geological Survey, stated that the formation that causes the falls is due to a change in the bedrock from soft crushed material to a hard tough l-ock, which the water finds it hard to cut into. Waterfalls are found more frequently in the upper than in the lower parts of a river and they may he expected in mountainous country like Bcuth Westland, The Upper Hokitika Liver, before joining the .Mungo des- | conds nearly 1100 feet in a series of cascades and falls, in the Kakapotalii’s granite gorges there are several small falls, evidence of a comparatively recent uplift in the foothills through which it flows.

A common form of the mountain type of waterfall in New Zealand is caused, according to an American geologist, hy a tributary coining from a higher level than the main river and tumbling into it over a precipice. The larger volume of water in the main river cuts, into the main valley deeper than the tributary stream cuts into the tributary valley. Hanging valleys, with their waterfalls, are very common in high glaciated mountains. Often when a mountain stream passes over hard beds of rock, it excavates more rapidly above it than below it. The water then “leaps from rock to rocTc, from ledge to ledge in series of sparkling cascades, varying in apearance from hour to hour will the changing conditions of light and shade, and always beautiful.”

Although waterfalls cannot be class: ii-ed in a definite way ordinarily, an el fort lias been made to classify tliei roughly in respect of their origin am to the causes that allow them to con timie. In one type a vertical front i. maintained. In the other type the ver tical front- is not maintain.d as tic. water retreats upstream. The world’: most- famous waterfall, Niagara, is re treating up-stream. It always has re tained its character as a vetrical wat erfall. Ages ago, Niagara River flow cd tranquilly through a fairly love’ plain. It suddenly dropped over tin bdge of mi escarpment seven miles be low the present site of the walls B'pray and frost wore away soft roc 1 and undermined hard rocks on top which resisted the water. This caused overhanging rocks to break, and piecesfell into the gorge.

The precipice formed hy the rive retreated up-stream, leaving behind deep gorge. With the knowledge of tli river’s processes available, it was hop ed that, by careful measurements, of present rate of retreat, it would In possible to estimate in centuries a least the. time Nature takes to dthis sort of work. The measurement 1 show ‘♦hat- Niagara’s average rate of re treat in 42 years was four feet a year In suite of this basis to work on, esti mates of the time required to carve out the imi-ge made by Niagara Riveare ,so wide that they give no reliabh idea to the ordinary person. The.es timntes range all the way from 700' years to 203,000, years.

Niagara Falls probably are the lies' illustrated in the world of a waterfai slowly but surely cutting back war: along a river’s course and leaving behind the gorge of which it is respon sible. The water goes over a precip ice of flat, hard limestone. Leu-oat' tin's are shales and sandstones, casih eroded. The lower, softer rock iloosened and large pieces of the hard er rock on the face of the precipice ar precipitated into the pool below. Ii this way the waterfall always is push ing itself back and lengthening it gorge. At present, the gorge is a bold seven miles long, between 200 and 80' feet deep. It is estimated that th quantity of rock removed by the w;u erfall’s forces would make a rampar’ 12 feet high and six feet thick al round the -earth at the equator.

Waterfalls and cascades result fron different characters carved by river : n their courses. In its descent a river operating on hard rock and soft rock may excavate its bed into platforms a' diflerent levels. Jf the descent fron one platform to another is vertical the water plunges in a volume, and r waterfall results. If the descent is steep, but not vertical cascades ma? be formed, their spray scattered by tin wind, and the sun’s rays throwing in to them rainbow hues. If the descent is gentle, not steep, and slopes instead of being abrupt, the current, flowing rapidly, is broken, and a cataract oi rapid is formed.

nen the velocity of a river is increased by a flood, its transporting power is increased almost incredibly. Ihe normal flow of the Sliotover Hive; Southland, is 300 cubic feet per second, clouds increase the flow to about. 9o(}(. cubic feet pei' second, Tilt* flow is in creased 27 times. In Hood, the Shot over carries lumps of rock that weigh nearly a ton as easily as in norma flow it carries pebbles that weigh 31b So great is the strength gathered in the fury of a heavy flood that a rivei in its anger may toss for many mile.* blocks and boulders it could not lilt m its placid moods.

“All the rivers Hut into the sea, yet the sea it? not full; into the placi whence the, rivers come thither the? return again.” This is so in most cas es, but many rivers run into lakes, like te Sehvyii running into Lake Ellesmere and Homo fade away in sandy deserts. In their haste to reach their base-lev-el, that is the sea or a large like, riv ers wear away all obstruction in then path sweep be!ore them all loose particles and rocks that obstruct, lfniming water lias little or no wearing effect It wears hard rocks away by carrying countless rasps in the form of sand and gravel. They are worn down h\ their work. By the rasping, chafing, and grinding bn a river’s floor, as Vrolessor .J. Park describes it angular blocks are rounded, boulders are reduced to pebbles, pebbles to sand, and sand to silt and mud. As the lighter particles arc carried in suspension, while the heavier particles are rolled along the floor, tile lighter ones travel further than the heavier ones.

Rivers are not always destructive. With drift, sand, and mud they fill the basins of lakes into which they How. Bivers often, by shooting enormous masses of material into the sea, carry on reclamation work. Sometimes, they iorin vast plains in this way. A great deal ol the material they gather from the land and discharges Jn the sea is spread over the sea’s floor by waves and currents, and every continent is fringed with wide submarine plains. I bey arc continental shelves, sloping gently further seaward to the hundred fathom lino, when the floor falls steep- *'•' It tln> waters receded to the hundred latliom line. Professor Park states, the continents would he fringed

by great maritiem plains