The main climate through which the Amazon River flows.

In Obidos Canyon, the river is restricted, and the Amazon River is only 65,438+0.6 km (65,438+0 miles) wide. The average depth of rivers below the medium water level is 6 1 m (200 feet), and the depth of most reaches in Brazil exceeds 46 meters (150 feet). From the upper reaches of Belem, several water depth records exceeded 9 1 m (300 feet). However, the Peruvian border is about 32 19 km (2,000 miles) from the Atlantic Ocean, and the altitude is less than 9 1 m (300 feet). The maximum width of the permanent riverbed of the river-free central island in the upper reaches of the Xingu estuary is 13.6 km (8.5 miles). During the flood peak, when the flood storage area is full of floods, the width of the river surface will expand to 56 kilometers (35 miles) or more. The average speed of the Amazon River is about 2.4 kilometers (65,438+0.5 miles) per hour. When the river overflows, the speed will increase greatly.

Due to the inconsistent rainy season in the whole basin, there are two rainy seasons in the upper reaches of the Amazon River every year, which are alternately influenced by tributaries originating in the Andes of Peru and those originating in the Andes of Ecuador. The former rainy season 10? 65438+ 10 month, while the rainy season of the latter is 3? July. This alternating influence on the farther downstream reaches just disappeared, and the two flood seasons gradually merged into one flood season. So from 1 1? In July, the downstream channel rose slowly, reaching the peak, and then fell back until 10 ended. In some places, the water level in flood season is higher than that in dry season 12? 15m (40？ 50 feet). Take four roughly equidistant positions as an example. The height of Iquitos is 6 meters (20 feet), the height of Tefei is 14 meters (45 feet), the height of Obidus is10/meter (35 feet), and the height of Belem is 4 meters (12 feet). The Negro River rises after the rainy season begins in February or March, reaches its peak in June, and then begins to decline with the Amazon River.

For thousands of years, the Amazon River has been winding on the vast flood plain without restriction, and a series of winding scars, oxbow lakes and recently abandoned old roads have appeared. Once the sediment is enough to reduce the velocity of the main stream of the river, the river will overflow the existing natural embankment and wash out a new river during the flood peak period. Similarly, in a few years or decades, new rivers will be silted up and rivers will be diverted again. However, most rivers flow along straight rivers. However, every flood season, sediment continues to fill the wide valley washed out by the river, and a large amount of sediment is deposited in the subsidence basin. Compared with the flow of river water, the flood plain is not very large. Siltation area is generally 19 wide? 48 kilometers (12? 30 miles), surrounded by steep cliffs. "terra caida" came into being where these cliffs were violently washed by the river.

The so-called blackwater tributaries of the Amazon River, including the Hingu River, Tapahos River, Negro River, Tefei River and Trombetas River, have little or no silt, partly because the soil from which they originated is white sand. The rivers of Taparos River and Xingu River are pale jasper, because they can't dissolve humus like Negro River. Where these tributaries flow into the main stream, freshwater lakes are formed due to the obstruction of river water, and their shape, width and depth are similar to those of the drowning bay (funnel-shaped estuary) at sea.

The influence of ebb and flow usually reaches Obidus Canyon, which is about 966 kilometers (600 miles) from the estuary. Before the spring tide, there is sometimes a surge called pororoca in the estuary, and the water level keeps rising, with an hourly rate of 1.6? 24 kilometers (10? 15 miles) and 1.5 miles? 4.5 meters (5? 15 feet) high unstoppable water walls spread out on the shallow water surfaces of the main stream and tributaries. In this case, the Amazon River cannot form a delta. It is estimated that150,000 tons of sediments are injected into the ocean from rivers every day, most of which are washed northward by coastal flows and deposited on the coast of Guyana. A group of islands and shallow beaches appear in the coastal zone with a length of 160 km (100 mile) from the northern headland to the southern part and inland to the northern edge of the Amazon estuary.

The Amazon river basin is located near the equator, with hot and humid climate and abundant rainfall. The annual average temperature is 25 ~ 27℃, and the annual average precipitation is mostly 1500 ~ 2500mm. It belongs to the tropical rain forest climate and is the largest distribution area of tropical rain forest in the world. The seasonal distribution of precipitation in the basin is relatively uniform, and the main stream water volume is compensated in different periods, which is abundant all year round and has little seasonal change. Every year, the amount of water injected into the Atlantic Ocean reaches 6.93 trillion m3, accounting for11/of the total water injected into the ocean by rivers all over the world. The average discharge of the estuary is 65,438+750,000 m3/s, the maximum discharge in flood season is above 220,000 m3/s, and the minimum discharge in dry season is also above 20,000 m3/s. The fluctuation of water level between the lowest water level and the highest water level in the river is more than 20m. Because the main stream and the right bank tributary of the Amazon River are located in the south of the equator, the change of river flow mainly depends on the right bank tributary, while the left bank tributary north of the equator only promotes the formation of the flood season and compensates the water quantity in the dry season. Due to the different rainy seasons in the north and south of the equator, there are two major floods in the Amazon River basin every year. The high flood season occurs from March to June, and the highest water level occurs in June, and its peak flow accounts for 40% of the total annual flow, and the secondary flood season occurs in 10 ~ 1 1. June-September is the dry season, which accounts for 14% of the total annual flow.

The Amazon river basin is low and flat, with small river gradient (about 1 cm/km) and slow flow rate. Flood discharge is not smooth in flood season, and the water level can be higher than the average water level10 ~15 m. The flood flooded the 80-250 km wide flat land (covering an area of about 50,000 square kilometers) on both sides of the middle and lower reaches of the floodplain, which lasted for several months. In Pingshui, the river width near Manaus in the middle reaches is also above 5km, 20km in the lower reaches and 80km in the estuary. Therefore, the Amazon River is also called "River and Ocean".

At the mouth of the Amazon River, due to the recent subsidence, the sediment brought by the river is washed away by the coastal waves, so there is no delta, and the estuary is a trumpet-shaped bay with a width of 320km, which provides favorable conditions for the ebb of the tide. Whenever the Atlantic Ocean tides invade, the seawater goes upstream, blocking the downstream channel, forming a tidal range of 1.5 ~ 2.44m (sometimes up to 4m). During the spring tide, a water wall with a height of 5m is often formed in the upper reaches, and its sound travels several kilometers away, with great momentum and spectacular scenery. Locals call it "Yamanu".

The Amazon River itself and most of its distribution originates from the tributaries of the Andes. Because the river carries a lot of sediment, the soil is rich in soluble nutrients, and the river is white and slightly yellow. The Negro River, which originated in the northern part of the Amazon River basin, is very old in geological age, with dark red (reddish) water color and strong acidity (pH value is 5. 1). When the Negro River joins the Amazon River near Manaus, two streams, one white and the other black, flow side by side for about 80 kilometers, black and white, and they are not mixed with each other. Originating from tributaries of ancient rock formations in the Brazilian Plateau (such as Taparos River and Hingu River), the river is clear.

The rivers originating in the Andes have the highest concentration of suspended solids (generally above 0.2kg/m3), indicating that these rivers are easily eroded and are called Baishui River. The content of suspended sediment in Heishui River and Qingshui River, which come from alluvial deposits on the plateau and mainland, is the lowest (less than 0.02 kg/m3, such as Negro River). Rivers mainly originating from alluvial deposits in the Andes and rivers with higher elevation and smaller slope in the upper reaches have medium suspended load concentration (0.05-0. 1 kg/m3, such as Prussia River and Yapna River). The distribution of suspended sediment in the Amazon River has obvious seasonality, which is caused by the different deposition, migration and cycle of sediments in its tributaries. The Amazon River carries about 362 million tons of sediment every year. In the Atlantic Ocean, which is 300 kilometers away from the estuary, you can also see yellow and turbid seawater.

The annual total precipitation in the Amazon River basin is 654.38+04.9 trillion m3 (or the rainfall depth is 2 150mm), of which 654.38+065.438+065.438+050 billion m3 is water vapor from outside the basin (mainly from the Atlantic side). Water vapor from the basin (local water cycle) accounts for 23% (3.4 trillion m3). About half of the precipitation (733 billion m3, accounting for 49.2%) returns to the atmosphere through evaporation; About 6930 billion m3 (46.5%) runoff flows into the Atlantic Ocean, and the remaining 640 billion m3 (4.3%) includes leakage loss (groundwater recharge) and the error in determining water balance.

The climate in the Amazon basin is warm, humid and rainy. Near the equator (not far north of the Amazon River), the length of the day, the length of the night, etc. There are often cloudless eyes in Wan Li at night, which is beneficial to radiate the heat received by the sun during the day 12 hours. The temperature difference between day and midnight is greater than that between the warmest month and the coolest month, so night is winter in the Amazon basin. In Manaus, the average daily high temperature is 32℃(89℉) and the average low temperature is 24℃(75℉). Occasionally, there will be cold periods, especially in the southern hemisphere winter. A particularly powerful air mass sweeps across the Amazon River basin from the polar regions to the north, resulting in a sudden drop in temperature. At any time of the year, after a few days of heavy rain, it is sunny, cool at night and low humidity. In the downstream area, the cool trade winds blow most of the year.

Precipitation is more important than temperature for the climate in this area. The moisture-filled wind from the Atlantic Ocean passes through South America and is forced to rise when it reaches the eastern slope of the Andes. In this way, the air is cooled and loses moisture through condensation. As a result, the heavy rain gathered into a river and flowed eastward from the Andes, forming such a vast Amazon river system. Over the lowlands, a large area of convective storms will produce a lot of precipitation.

According to different rainfall conditions, the Amazon region can be divided into three climate types: the first one occurs in the mouth of the Amazon River and the west of the basin, with an average annual rainfall of more than 2000 mm (80 inches) and a uniform annual rainfall distribution; In some years, the precipitation can reach twice the normal precipitation, and in some years, it can be dry for a long time without rain. The second includes most parts of the Amazon, and there is a season when the precipitation is particularly low, but it is not serious enough to affect plant growth. The third type includes the southern margin of the Amazon basin, where the climate gradually changes to the central and western Brazil, and the southern hemisphere has a more obvious dry season in winter.

In the dry season, the prevailing wind direction is east-northeast to east-southeast, and it is gentle wind in July and August, but in the rest of the dry season, gusts sometimes reach strong winds. This season is the best time for tourists to row upstream or downstream.