When I was searching for some answers I found an article in Fiji Daily Post - with some explanation of how man has changed the rivers by removing rocks and gravel etc. Here are parts of that article - written a few months before the current disastrous floods in Nadi and other places.
The organisation - NatureFiji-MareqetiViti - that produced this article seems to be one of the finest of the non-government organisation in Fiji and their website is well worth a good look.
Destruction of Fiji’s Rivers & Streams
Fiji Daily Post
Date: September 9, 2008
Fiji’s rivers and creeks are a little-recognised resource of great subsistence and sustenance value for the traditional Fijian way of life and the majority of rural dwellers. They are also of considerable biodiversity interest, the recognition of which has only come about in the last few years.
Increasingly, however, the greatest locally recognised value of rivers is as a source of gravel and rocks for the construction industry and consequent “gravel rental” or lease rights for adjacent landowners.
The very rapid and unsustainable manner in which gravel and boulders are being extracted is causing major ecological changes in many of Fiji’s rivers and streams, and major changes in flow rates and volumes.
The economic impacts of these changes are beginning to be felt in the form of increased and more extreme flooding, increased dredging needs, and the undermining of the foundations of bridges, Irish crossings, river walls, culverts etc.
The economic costs to the nation of increased severe flooding, increased dredging and the replacement of riverine infrastructure will be enormous, and will be paid for by taxpayers - not the extractors of gravel and boulders. There is an urgent need for the extraction of sand, gravel and boulders to be brought under control and undertaken only at sustainable levels, with the bulk of these materials being sourced from quarries and gravel pits.
Traditional Values of Rivers & Streams:
Unpolluted rivers and streams have traditionally been an essential component of traditional rural lifestyles.
Apart from wild pigs, rivers and streams have in the past provided the bulk of essential dietary protein for the vast majority of inland Fijians in the form of fish, prawns and eels. The loss of these is a major detriment to rural lifestyles.
Similarly a constant supply of clean water has been an expectation of rural living and one of the important determinants in the location of villages. Rivers and streams have always provided drinking water, but have also been important for washing and bathing, as well as for livestock needs.
Until very recently, including the 1993 National Environment Strategy, it was believed that Fiji’s freshwater biodiversity was limited and uninteresting with minimal endemism (i.e. no species that are only found in Fiji). However, in the last five years research has shown that this assumption is very wrong.
There are around 143 species of fish, invertebrates and plants in Fiji that are known to spend at least half of their lives in freshwater.
Some 132 of these are native species while the other 11 are known well established populations of introduced fishes.
There are at least ten known species of native fish that are only found in Fiji’s rivers and streams (i.e. endemic species).
Most of these endemics are widespread throughout the high islands, however some species are only known from single streams (for example, waitavala and tavoro on Taveuni island).
There are also two additional suspected undescribed endemic pipefishes from the genus microphis that inhabit streams in Kadavu.
There are at least five native fish species that could be considered endangered, these include three species of gobie whos restricted habitat and small population size render them highly vulnerable to extinction.
The two other species are listed on the IUCN Redlist of endangered species and are extremely rare, they are the Otomebora Mullet (Liza melinoptera) and a species of Grouper called Epinephalus lanceolatus.
River sand, aggregate and rocks have become a very common and cheap source of material for Fiji’s construction and road building industries, indeed they have fuelled the construction industry for the past half century.
With Native Land Trust Board (NLTB) providing “gravel rental’ lease rights or Open Licenses to commercial operators in exchange for fees, landowners adjacent to rivers have enjoyed cash windfalls.
However, much of this land that is being leased and mined for gravel is not on native land but is in fact legally owned by the State.
Rock, gravel and sand production is a natural process in every catchment system and there is a sustainable level of extraction at which little or no impact arises. However, exceeding that extraction level can lead to major impacts and these are all too clearly seen in many if not most rivers near Fiji’s urban centres.
Removing boulders and rocks dramatically changes the hydraulic characteristics of a river – generally Fiji’s rivers, in their natural state, are a mixture of quiet pools or stretches of slow moving water, interspersed with whitewater rapids of variable length and water velocity.
This is the natural order of rivers and Fiji’s native species are adapted to this alternating river passage and habitats.
Excessive extraction of river material eliminates habitat differentiation (pools and rapids) and the river takes on the characteristics of a smooth, culvert with a lowered river base course.
This has the following consequences:
1. Different river habitats are lost (pools, rapids, stretches etc.) with disastrous consequences for Fijian aquatic fauna i.e. loss or a great reduction in prawns and Ika Droka;
2. With the river reduced to a ‘culvertlike’condition, floods are not ‘held up’ by a varied river topography, and so proceed down the catchment at a much faster rate. If this coincides with a high or rising tide, then much larger floods are experienced at the river mouth. This is what has been happening increasingly in Nadi over the past two decades and occurred at Labasa during Cyclone Ami. Mistakenly attributed to ‘climate change’, increased flooding for many of Fiji’s rivers is something which is entirely of our own making;
3. Fine gravel, sand and silt are delivered much faster to the mouth of the river where they settle out and impede navigation and may increase flooding. Costly dredging is required more frequently to mitigate these effects; and
4. Lowering the base course of rivers, undermines those structure which were built when the rivers were at a higher level i.e. bridges, Irish crossings, culverts, irrigation off-takes, and river and flood protection walls. Undermining such structures means that they will not withstand the design forces they were built for and will sooner or later have to be replaced.
The first of these impacts effects biodiversity values, but very importantly the ability of subsistence dwellers to obtain a major portion of their dietary protein and to enjoy an unaffected rural lifestyle to which they are accustomed.
The last three of these impacts pose significant risk and financial implications for downstream dwellers, and major financial implications for the State, which will ultimately be borne by the taxpayers.
The current generation, in particular the construction and tourism industries, are mining a resource at very little cost to themselves, the detriment of which will be felt by future generations.
That the State allows this extraction from the nation’s rivers, without any consideration of the financial or social implications, is a classic example of unsustainable resource management.
Removal of large boulders and rocks from the upper stretches of rivers can be especially damaging. This has occurred to exhaustion on the Sabeto River.
Much of these rocks went to the Denarau Resort, the Sonaisali Resorts and the Wailoaloa Fantasy developments the irrational issue here is that angular quarried rock is much superior as an engineering material than rounded boulder rock for the armouring of sea walls.
Considering the massive price that is being paid by Fiji’s natural environment as well as its current and future generations, it seems perverse that the use of these rocks is often purely decorative.
NatureFiji-MareqetiViti (NFMV) is a local conservation group whose mission is to enhance biodiversity and habitat conservation, endangered species protection and the sustainable use of Fiji’s natural resources through promoting collaborative conservation action, awareness raising, education, research and biodiversity information exchange.