natural vegetation is removed to make way for agriculture or perhaps decimated by overgrazing, accelerated erosion often results.

Rain that is not absorbed by the soil run off as sheet flow sheet flow is a thin layer of water that moves down slope without being confined to channels. This thin film of water can cause considerable erosion as it removes fine grained surface materials such sheet erosion is an important degradational process in certain areas. Especially in bare slope which is unable to absorb water rapidly. Continued run off causes initiation and growth of small channels called rills that may merge into larger brooks or creeks, and these in turn coalesce into more permanent streams. This water carries sediment with it, and both water and sediment become part of the river system that is in the process of shaping the regional landscape.

    A river system consists of a trunk river joined by a number of tributaries. These branch streams diminish in size the farther they are from the trunk river, the complete system of the trunk and its tributaries forms a network that occupies a region known as a drainage basin.

Drainage basins of zunoz are separated by Sultan sanjar mount, Gharannok Darreh, Adineh, Diraghah, Gerd Ovol, Gebla Daghi, water from the western slope of sultan sanjar flows into the zunoz river, water on the eastern slope of this mountain flow into the another drainage basin.

To assess the erosional activity of zunoz river based on quantity of sediment that passes a cross-section of the zunoz river near its mouth another set of measurement identifies every stream segment of the network and the sedimentary load carried by each. We can tell, the larger a drainage basin is, the larger will be the sediment load carried out of it at the mouth of its trunk river. But this is not always become true. These are conditions that influence the rate of erosion. Obviously, the amount of precipitation is a factor. The more water available, the more erosion can take place. But the cover of vegetation also plays an important role because it inhibits erosion. The relief in the drainage basin is important as well. In a basin where the relief is generally low, slopes are less steep. Water moves slowly, and erosion is less active than in a basin where relief is high. Another factor is the underlying lithology. Soft sedimentary and weak metamorphic rocks are degraded much more rapidly then hard crystalline rocks. We must also consider human impact to be a factor influencing zunoz river basin erosion rates.

The main functions of zunoz river can be grouped into three stages:

Erosion (degradation) transportation (transfer) and deposition (aggradation). Before we examine these processes let us to consider what happens to zunoz valley itself. It changes continuously. zunoz river can be evolved through stages of youth, maturity and old age. Three model stages in the life cycle of zunuz river. In the youth, the river’s energy concentrates on deepening its valley. In the eastern part it is in a v-shaped. In maturity, river’s energy is directed laterally, and a flood plain forms in the western part of zunuz, you see maturity’stage in the western part about 6 kilometers of zunoz .In the old age, floodplain is much wider near Marand-Jolfa road. zunoz river flows rapidly, even wildly in mountainous areas in the eastern part of zunoz and many tributaries join to it in the eastern part and rush down slope in deep valleys, it resembling a v-shape. Near Sildrim of  Charaza area.

Rivers are the most important and efficient of all agents of erosion, continuously modifying their valleys and changing the landscape. River erosion takes place in three ways: hydraulic action, abrasion and corrosion.

1- Hydraulic action: It refers the work of water, as it dislodges and drags away rock material from the valley floor and sides.

2.- Abrasion refers to the erosive action of boulders, pebbles and small grains of sediment as they are carried along the river valley. Gravel and sand-sized particles tend to scour the valley bottom, wearing it down while evening out the rough edges.

3- Corrosion is the least important form of erosion by river. It is the process by which certain rocks and minerals are dissolved by water.

Splash erosion‏  is the detachment and airborne movement of small soil particles caused by the impact of raindrops on soil.

Sheet erosion is the detachment of soil particles by raindrop impact and their removal down slope by water flowing over land. Once the rate of falling rain faster than infiltration, run off takes place.

Rill erosion  refers to the development of small ephemeral concentrated flow paths, which function as both sediment source and sediment delivery systems for erosion on hill slopes.

Water erosion

The rill bed surface changes as soil erodes, which in turn alters the hydraulics of the flow velocity, depth, width, hydraulic roughness, local bed slope, friction  slope, and detachment rate time and space variable functions  the rill evolutionary process.

 

Transportation by rivers

Erosion and transportation go hand in hand.

The materials loosened by hydraulic action and abrasion as well as dissolved minerals, are carried downstream.

Along the way, different transportation processes are dominant. There are four processes at work when a river transports its load: traction saltation, suspension, and solution.

 

1- Traction: Traction refers to the sliding or rolling of particles along the river bed.

2- Saltation: Speed of the wafer lifts these fragments off the river bottom. Saltation, therefore, is a combination of traction and suspension the particles make contact with the stream bed, but they also are briefly suspended as they move along.

3- Suspension: Very fine sediment, now ground down to silt and clay sized particles, is carried within the stream by a process known as suspension. When suspension dominates, water becomes muddy.

4- Solution: Some rock material dissolves in stream water and is carried downriver in solution.

a. Stream power or stream capacity refers the maximum load of sediment that a stream can carry with a given discharge or volume of water. It is obvious that a stream with a large discharge has a larger capacity than a smaller stream.

b. The velocity of a stream, therefore, is a critical factor in its ability to erode and transport. As a rule, water velocity is greatest in the middle of the channel. But near the bank and on the river bottom the velocity is slowest because friction between water and rock slows the water’s motion.

The gradient or slope, is the key factor in a stream’s velocity whereas, a wide, shallow channel generates too much friction and reduces both velocity and erosional capacity.

 

Stream floods

Stream flow volume varies through time as a result of changing water from adjacent upland areas and tributaries. Abnormally high rainfall or abrupt seasonal snow melt can produce a flood in the trunk river. Flood occurs when a stream overflows its channel.

Flood plains are built by successive floods as sediment is deposited as alluvium; each flood adds more sediment to the plain.

 Base levels

We can learn much about the behavior of streams by studying their longitudinal profiles, their downward curve from the interior highlands to the coast.

Base levels-the level below which a stream can not erode its bed.

Absolute base level lies no more than a few meters below sea level.

Local base level, a stream that originates from the mountains and flow into lake, the  surface of lake is its local base level.

Energy and work in a river system

The water generates kinetic energy as it flows down slope.

The motion of water in a river represents the transformation of potential energy into kinetic energy to carry its local and move itself. When the water reaches its base level, there is no more potential energy available.

Graded river represents a balance among valley profile, water volume, water velocity, and transported load. Many factors are involved in tendency for a river to attain a graded state, and these factors may be classed as either independent, semi dependent, or dependent.

The consensus is that a graded stream is one in which, over a period of time, stream slope, channel characteristics, and flow volumes are delicately adjusted to provide just the velocity required for the transportation of the load supplied from the drainage basin.

A graded stream is that any change in any of its controlling factors causes a displacement of the equilibrium in a direction that tends to absorb the effect of the change.

Rivers degrade, transport, and aggrade, here we have studied the processes of river erosion.

The geologic work of Zonouz river consists of three closely interrelated activities, erosion, transportation, and deposition. River erosion is the removal of mineral material from the floor and sides of channel, whether bedrock or regolith. Transportation consists of movement of the eroded particles by dragging along the bed, either the largest or the smallest, such as boulder, gravel, pebble, cobble and sand or suspension in the river like silt and clay.

There are four processes in transportation when a river transports its load

.Traction: refers to the sliding or rolling of particles along the river bed.

Saltation: indicates the speed of the water lifts these fragments off the river bottom.

Suspension: In this stage, very fine sediment ground down to sit and clay sized particles is carried with the stream.

Solution: On this process, some rock material dissolved in stream water and is carried down river in solution.

Erosion, transportation and deposition are three phases of a single activity which occur in this river. The larger materials are carried in the upper part of river where stream bed is steeper and the water has more force.

Erosion (degradation) is the main activity of river which occurs in the upper parts whereas deposition (aggradation) takes place in the lower parts of the river.

Rainfall, weathering and type of soil are three factors that have caused the badland of zonouz was formed. (figure ….)

It should be mentioned that water erosion on the bare slopes are extremely more intensive than gentle and vegetated slopes.

Landslide, soil creep and fall movements are considered in zonuoz valley.

Soil creep appears to result from the altenate freezing and thawing of soil particles or from alternate periods of wetting and drying.

Fall, or the downslope rolling of pieces of rock that have come loosened by weathering. (Figure ….).

Water erosion which occurs in zonouz valley can be seen in three ways: hydraulic action, abrasion and corrosion:

  1. Hydraulic action: It refers the work of water, as it dislodges and drags away rock material from the valley floor and sides.
  2. Abrasion refers to the erosive action of boulders, pebbles and small grains of sediment as they are carried along the river valley.
  3. Corrosion is the least important form of ersion by river. It is the process by which certain rocks and minerals are dissolved by water.

Three significant factors should be considered in studying of zonouz river.

  1. drainage basin, the region occupied by a complete stream system formed by the trunk river and its tributaries (as known as watershed).
  2. channels: the rain that is not absorbed by the soil runs off as sheet flow. Sheet flow is a thin layer of water that moves down slope without being confined to channels. It removes fine- grained surface materials.
  3. Alluvial fan: a fan- shaped deposit consisting of alluvial material located where a mountain stream emerges onto a plain.

The larger materials that river can not move are remained in upper parts of river, where as smaller grains such as pebble, grovel, sandstone siltsand carried to the lower part. (See the figure ….) Zonouz river which derives form slotan sanjar mountain in the east, flows to the west about 25 kilomerters.

It passes through a deep valley. (Figure … ) orchard gardens lying on both sides of river are irrigated by this rivers.

The average debit of river is about 3.4 cubic meters per second annually. In June, July and August the bed of river is completely dried.

The movement of materials related to the speed of river. Cobble, gravel, boulder can be transported when the velocity of river is exceeded. Fine- grained material like siltstone, mudstone are transported when velocity of river decreases.

The main agents which affect on the erosion of zonouz valley are as follow:

 Steep of mountain slopes

. Composition of soil

. kind of soil

.  Variety of relief and rugged

. bare slopes of mountain

. shape of mountain

. Human’s effects

The high rocky cliff is located on the left side of zonouz river which extended to the east about 3 kilomerters length.

The cliff is made of granite stone. It is an impenetrable stone. The erosion can be seen here in the form of fall movements Free fall, or the down slope rolling of pieces of rock that have become loosened by weathering. Such rock fragments and boulders usually do not go far, ending at the base of cliff. The accumulation of rock fragments, large and small, at the foot of the slope. Rockslides and falling of stones are a constant hazard for the gardeners (Figure …)

In the left bank of river near Sildrim, the landslide has occurred since 1955 and are being continued to day. The soil of slope is formed mostly by shale.

 

1. Mechanical weathering also called physical weathering, takes place when rocks are broken down without any change in the chemical nature of the rocks.

Mechanical weathering can be seen in three states: Frost action, frost creep and frost wedging.

2. Chemical weathering it involves the change in the composition of rocks, often leading to a breakdown in its form. Type of chemical weathering are:

- dissolution

- Hydration

- Hydrolysis

- Oxidation

- biological

- Carbonation

3. Biological weathering It is through the break down of  rocks and the accumulation of a layer of minerals that plants can grow plants whose roots contribute to the weathering.

4. Human activity contributes to various forms of weathering in a number of ways:

1- by polluting air with various substances.

2- by Quarrying and mining

3- by farming