Pocesses and the Resource Base

River Flooding

The portion of the River Indus that flows through Sindh  is one of the very few rivers in the world that flows on a ridge and changes its course frequently. The river flooding depends on peak discharges in individual rivers that join the River Indus. It also depends on the discharge of all or most of the rivers joining the Guddu barrage at the same time.

A discharge of about 300, 000 cusecs in the Indus River at the Guddu and Sukkur barrage is adequate to inundate the low-lying areas of the riverine tract in Sindh. The discharge of 500,000 cusecs floods the intermediate areas in the riverine forests. The discharge of 700,000 cusecs floods the high-lying areas. The entire area is flooded when the discharge is about one million cusecs. The flooding may start as early as May and recede by the end of August.

Dhand and Dhoras

The River Indus meanders changing its course frequently, learning behind active or non-active rivulets, which are known as dhands and dhoras. Those retaining substantial quantity of water for quite some time harbour fish, turtles and tortoises. They are a  source of water for irrigation of trees and may also be used for agricultural purposes. Lai (Tamarix aphylla and Tamarix dioca) can be seen growing along the river banks. This provides a good habitat for wildlife such as hog-deer, wild boar and black partridge. The banks and the adjoining areas are good for the regeneration of obhan and babul .

Erosion and Accretion

River flooding causes large tracts of land being eaten-up on one bank of the river and corresponding area of land being created on the other. A river bank is eroded more when water is receding than when it is rising. In the initial stages, ' kachhos (new lands created in the riverine tract) are unfit for regeneration, but gradually they become stable and suitable for rehabilitation. Many forest areas have gone under cultivation and are in private ownership because of the principle of m ohag i.e. the right of frontage that enables the adjoining farmers to rightfully claim the ownership of the newly created land.

Soils

The Sindh plain has a sedimentary formation of layers of varying soil textures, one over the other. The basic layer is of sand of the eastern desert or the sand from the slopes of the western calcareous Kirthar formation. The soils of the riverine forests of Sindh are sandy, loamy and loamy clays.

The sediments carried by the River Indus since 1902 have been measured . On an average, Indus brings 8,500 million cubic feet tonnes of sediments every year. This is equivalent to 200,000 acre feet. This is sufficient to silt up the entire area under the three barrages in Sindh (Guddu, Sukkur, and Kotri) to an average depth of 3 inches.

The high-lying areas are created by sedimentation, caused by consecutive high floods, inundating the high grounds. High floods bring with them coarser particles, often in the form of sand mounds and deposit sand bars.

Soils of scroll areas

The confinement of River water within embankments ( bunds ) has increased its capacity to erode the formation that had made up its banks. This has resulted in degradation of soils in the riverine tract. In its meandering activity, old formations were eroded and its finer particles were carried downstream, the coarser particles made up the fresh formation downstream on the other bank. Thus any post-confinement formation has a lighter composition of soils as compared with the previous formations. The finer particles have made new lands in the delta region. The scroll areas are now built up of coarser sediments. The humps between the two scrolls are sandy. Narrow belts along the scrolls are sandy loam to loam. The water holding capacity of the scroll areas is considerably low due to coarser composition of its soils.

Sand bars

The sand bars have variable length, breadth and height. They extend  from 2 to 5 miles continuously along the river fringes, and have maximum height of 4 feet. These sand bars are overlaid on the soils which may have been fertile and stable and thereby create conditions unfavourable for regeneration and establishment of new vegetation.

In certain areas the established tree stands on old land formations get buried with sand bars. If these stands are dense they may survive to attain the commercial size for exploitation by creating humid micro-climate on the floor. If the canopy is open, the forest floor gets dry is lethal to the growth of tender vegetation including Babul . If sand is removed to expose the buried stems  of the trees before felling, the yield of timber increases substantially. Subsequent regeneration of such areas after felling of established stands is difficult.

The role of the River in supplying water and changing the configuration of land and soil regime has an overall effect on the vegetation the area can support. The changes brought about by the river in the past are unfavourable to soils, water regimes and vegetation. As a result, the riverine forests have suffered from such adverse changes.

Topography

The differences in topography make these plains flood differently.The areas between bunds are very high-lying areas, which are seldom flooded;high-lying areas flooded once in 4-5 years;intermediate areas which are subject to flooding every 1-3 years;and low-lying areas that are flooded annually.Therefore, topography determines flooding, species and management regime.

The ground water-table in high-lying areas is deeper than 3 meters. In case of agriculture and plantations lift pumps or tube-wells are used to lift water. Water could also be lifted from dhands and dhoras.

The Intermediate areas are inundated up-to four feet in depth.The depth of the ground water is less than 3 meters. These areas are best suited for growing desirable tree crops of better economic value. For raising plantations in these areas wells are dug and used for hand watering. Water is lifted manually, by the labourers who carry two tins of water attached to a pole over their shoulders. This water is then poured into each boat-shaped or circular pit, which contains a single plant. The open wells are one meter wide and about 3 meters deep. Two hundred plants are irrigated from a well. Due to low discharge from dug wells the utility of this system for large scale plantation is limited. However, this system is suited to poverty reduction programme as jobs can be  provided to many people.

Low-lying areas are subjected to annual flooding. However, the rate of survival of plants other than lai is low as the plants remain under water for a long period and do not survive. Only those plants survive which grow sufficiently high before the next flood.

The topography of the floodplain is also vulnerable to changes. Considerable changes occur in the ground level due to huge quantities of sediment being deposited especially in low-lying and intermediate areas.

The topography has a very important bearing on the management of the riverine forests. The practices in riverine forests of Sindh and Punjab in this context are summarised in Table 2 below.

Table: 2; Management of Eco-zones In Riverine Forests 

Source:Riverine Forests
Forestry Sector Master Plan- Pakistan-September -'91

Ground Water Table

Comprehensive ground water survey of the entire river belt has not been done so far. However,  Mr M.H.Penhwar has made the following assessment:

• The ground water in the river belt is sweet except for the river belt in Thatta District and on the right bank from Sehwan to Jherruk.
• The river belt on the right bank from Bagarji forest to Soi forest is reported to have sweet ground water up-to 50 or 70 feet depth. This water is not copious. Water is saline below 70 feet.
• In the river belt on the left bank, ground water of good quality is available in large quantities from north up-to Kotri Barrage.
• On the right bank, ground water in previously active river-beds within last 50 years is reliable in quality. Elsewhere in that tract the ground water is not copious and its quality would probably be unreliable. 

Forests - Types and Species

The riverine forests owe their existence to flooding by rivers. After the construction of embankments ( bunds ) some of these forests  were left out of embankments but sluices were made in the embankments to allow flood water to reach these forests. With the construction of barrages (Guddu, Sukkur  and Kotri) on the River Indus, canals displaced sluices for irrigation of most of these inland (outside of protection bunds ) forests. There is yet another category of riverine forests, mostly in the   Province of Punjab, that has been raised as irrigated plantations.

In Sindh , the riverine  forests  grow along the River Indus, between the banks of the active river and the edge of permanent flood plain. The  rich alluvial soils support crops of Acacia nilotica (babul) with Populus euphratica (bahan or obhan),Tamarix aphylla, Tamarix dioica (Lai) and Prosopis cineraria (Kandi).

The species growing in the sand dunes of the riverine sand-dune scrub include; Zizyphus mauritiana, Prosopis spicigera and Prosopis glandulosa. Pennisetum dichotomum, Aerua javanica, Leptadenia spartium, Saccharum bengalense,

The main tree species growing in the riverine forests of Punjab are Dalbergia sissoo (Shisham) and Morus alba (Mulberry ). The undergrowth usually consists of Tamarix dioca (Lai), Zizphus Mauritiana, Acacia farnesiana, (Kikri), Saccharum munja, (Kana)  and Saccharum spontaneum (Kahi).

After substantial reduction in flooding due to abstraction and extraction of water upstream, very high-lying areas, high-lying areas as well as some of the intermediate areas are faced with non-occurrence of flooding. Irrigated plantations are being raised in some of these areas with tube-wells or with water lifted from dhands and dhoras.