Question

give the reason the river are Swift at their sources and slow down in their middle and old courses give me the answer in short

Answer 1

In the middle course of a river the gradient decreases (it flattens out) and the discharge increases. This is due to the fact that many more tributaries have joined the main channel, leading to an increased volume of water, as well as the fact that the high level of gravitational potential energy which was found in the upper course of the river has been converted to kinetic energy.
This enables the river to erode laterally as well as vertically, thus widening the river channel. The river also starts to deposit load due to variations in energy in the channel cross-section and longitudinally (caused by differences in geology, gradient, land use and weather conditions). River energy fluctuations along its course leads to a dynamic environment whereby the form of the river can change season by season.

Many landforms are formed in the middle course as a direct consequence of the relative energy balance changing, resulting in both erosion and deposition. The most obvious example of this is a meander.

Meanders

Meanders are the characteristic bends in a river that are found all along the length of a river but particularly in the middle course. They can vary in size, but generally have deposits of different sized sediment on the inside of the bend called a slip-off slope or point bar, and a concave section on the opposite bank of the river on the outer bend called a river cliff or bluff.

The initial cause of meandering rivers is related to the way in which different types of flow within a river:

laminar flow – horizontal movement in a plane.
turbulent flow – horizontal and vertical eddies in the water sometimes producing mini-whirlpools
helicoidal flow – corkscrew movement of water which is thought to transport eroded material downstream from one meander bend to the next
These, combined with the natural sinuosity (the movement of a river in an S shape across the landscape) of water leads to different rates of erosion and deposition. It is thought that the frictional drag exerted on the water by the riverbed and banks causes lower layers of water to move more slowly and in a sinuous manner. They interfere with upper layers of water and cause them to move in a more exaggerated pattern of sinuosity. Inevitably, once sinuous flow is established it causes the river to collide with the banks and results in erosion.

The increase in discharge typically found in the middle course leads to an increase in the rate of erosion (both vertical and lateral) and the formation of pools in the channel. The rate of formation of pools increases in times of higher discharge and leads to more efficient movement of water. In addition, erosion of the channel causes there to be an increase in the availability of material being transported. This material is often deposited further along the channel in less efficient sections with a lower velocity, to form riffles. Riffles reduce the efficiency of channel flow still further by increasing the wetted perimeter (the length of channel in contact with water) and consequently increases friction. This reduces the velocity of the water and leads to further deposition. Coarse particles in riffles are more angular and generate greater flow resistance than the fine particles in pools so more energy is expended as water flows over them (the water is shallower). This, in turn, encourages further deposition. Pools have a shallower sediment gradient so less energy is expended by the deeper water flowing over them, further increasing channel efficiency.

Pool and riffle sequences cause changes in the efficiency of a river channel which, in turn, causes changes to the way in which the water moves within the channel. The swing of flow caused by riffles forces the line of greatest velocity (the thalweg) to the outside of the channel where it starts to erode the bank by hydraulic action and abrasion.