Firstly, the region we are dealing with is known as the Amazon Basin, so it is important to understand what a basin is

Firstly, the region we are dealing with is known as the Amazon Basin, so it is important to understand what a basin is.

Basin[1]: A topographic rock structure whose shape is concave downwards.

Basin[2]: 1. n. [Geology] A depression in the crust of the Earth, caused by plate tectonic activity and subsidence, in which sediments accumulate. Sedimentary basins vary from bowl-shaped to elongated troughs. Basins can be bounded by faults. Rift basins are commonly symmetrical; basins along continental margins tend to be asymmetrical. If rich hydrocarbon source rocks occur in combination with appropriate depth and duration of burial, then a petroleum system can develop within the basin.

So basically, the Amazon basin is shaped like a bowl, not an upside-down bowl or a plate. The Amazon is also a drainage basin,

Drainage Basin[3]: Land surface region drained by a length of stream channel.

since the Amazon River runs through the sediment eventually draining into the Atlantic Ocean. This brings up another definition of the basin due to the sediment filling it and deposited by the river which drains it.

Sedimentary Basin[4]: 1. n. [Geology] A depression in the crust of the Earth formed by plate tectonic activity in which sediments accumulate. Continued deposition can cause further depression or subsidence. Sedimentary basins, or simply basins, vary from bowl-shaped to elongated troughs. If rich hydrocarbon source rocks occur in combination with appropriate depth and duration of burial, hydrocarbon generation can occur within the basin.

Naturally the Amazon Sedimentary Drainage Basin is in a state of net deposition, building the flood plain higher up more during each rainy season than erodes during the remainder of the cycle. However, this is not to say that erosion is not a problem in the Amazon. In some of these regions, the deposition is beneficial, bringing fresh mineral nutrients down from the Andes and the surrounding uplands, however, this accumulation is not always ideal. It is easy for these accretions to err on the side of too much sand or too much clay and a lack of mud. This brings up another question, what exactly is the Amazon soil composed of? Let us leave the matter of erosion for a moment, to address the composition of the soil itself. There are four horizons, or essentially levels within

Horizon[5]: (1) A surface separating two beds in sedimentary rock. (2) A layer within a soil showing unique pedogenic characteristics. Four major horizons are normally found in a soil profile: A, B, C, and O. (3) Point at which the visible edge of the Earth's surface meets the sky.

the soil, O on the surface is the fresh organic material beginning to decay, below that is A with mineral nutrients and organic particles (this is only one inch thick in the Amazon). Next down is the B horizon, with primarily rock particles, and the nutrients that have leeched out of the A horizon, and the final stratum is the bedrock of the C horizon.[6] The bedrock of the C horizon, which is the foundation of the Amazon’s Basin feature, is Precambrian in nature. The B horizon however has precious few rocks, because of the corrosive effects of a journey down from the Andes precious little makes it to the basin itself (curiously, rocks are in such short supply in the Amazon that for many native cultures they are a form of currency). Instead, the B horizon is almost entirely composed of soft sedimentary clay, running as deep as four kilometers in many places. This clay is inferior for several reasons, first it is rather unstable, and allows the sediment to under go compression in severely irregular patterns (this has made large scale construction projects very difficult in the Amazon)[7]. Secondly, clay is a natural sealant, it makes the osmosis of water ions and nutrients virtually impossible, so when the A horizon is only an inch or so thick, there is virtually nothing accessible to foster growth bellow that.

This leads to radically shallow root systems in the flora, which has inherently negative effects on the hardiness of these trees. It also intensifies the gravity of the erosion issue, for when there is only one or two inches of accessible fertile soil, one can not afford to loose any of it. The effects of leaching on soil nutrients were already mentioned, but a second form of erosion is far more measurable; sheet or surface erosion is what happens when soil is simply washed away, in this case the scarce and valuable layer of top soil is victim to these forces. Normally this does not happen, because the root system (shallow as it may be) is like a safety net, keeping the fertile soil locked in place, as well as the extensive canopy limiting the amount of actual water reaching the forest floor in its liquid form. As such it would seem that erosion is not a matter worth looking into, but unfortunately, the flora system which protects the rich productive soils is not always present. Due to clearing and the encroachment of farms, mines, roads and other human uses of he Amazon land, there are many patches of relatively unprotected soils both on the edges and within the rainforest. Erosion in these deforested areas, even if there is agricultural vegetation upon it, is drastic, irreparably depleting the existing nutrients in only a season or two. This makes the reclamation of deforested land a loosing battle, but even worse than that are the side effects on adjacent forest. Healthy forests sharing a periphery with these damaged treeless areas are going to fall victim to the radiating forces of erosion, and the grasping draw of nutrient leaching.

Why are these erosion effects an issue? It is a very basic matter, if the nutrients are either removed made inaccessible, then the flora cannot prosper. By definition a rainforest must have some sort of forest, and without flora, that is quite difficult. Secondarily, erosion explains why reforestation is such a costly and intensive process, it is like administering electrical shock to recover a victim of cardiac arrest: the amounts of artificial fertilizers and soil enhancers required in such a sort time are staggering, and any foreign substance in such volumes will significantly shock the system.

What can be done about these issues? Erosion is an indicator, it can be used to signal what is happening upriver, or in remote regions that are not easily accessible to direct monitoring. By correlating the sediments found in transport away form the site of origin, one can pinpoint areas where deforestation and nutrient seepage are occurring. By knowing what we do about the causes of erosion, strategies can be formulated to retain the fertility of the land, even if the native root network is removed. Development of these strategies is essential if there is any hope to restore or repair sections of denatured forest.

[7] “Where are the Rocks?” Tropical Rainforests: The Understory. http://www.mongabay.com/05where_are_the_rocks.htm, 1996-2002.