Understanding how rivers behave based on sediment load is key to grasping fluvial geomorphology. Rivers constantly interact with their channels, either eroding material (degradation) or depositing it (aggradation). This balance depends significantly on the relationship between the sediment load the river carries and its capacity to transport that sediment.

River Sediment Deficit Causes Degradation

A sediment deficit occurs when the amount of sediment a river needs to carry its load exceeds the actual amount of sediment available. This typically happens when the water flow has more energy than required to move the available sediment.

When there's a deficit, the river has excess energy. To balance this, it uses its energy to erode its own bed and banks, picking up sediment particles. This process lowers the riverbed elevation over time. Rivers that are actively lowering their beds due to a lack of sediment supply are described as being of the degrading type.

Understanding River Behavior Types

Let's look at the different types of river behaviors mentioned:

• Meandering type: This describes the shape of the river channel, characterized by sinuous bends. While sediment transport influences meandering, a deficit specifically points towards erosion, not just the pattern of bends.
• Aggrading type: This is the opposite of degradation. An aggrading river builds up its bed because the sediment supply is greater than the river's transport capacity. Sediment is deposited, raising the bed level. This occurs when there is a sediment surplus, not a deficit.
• Degrading type: As explained above, this occurs when the river erodes its bed due to insufficient sediment supply compared to its transport capacity. A sediment deficit directly leads to this condition.
• Subcritical type: This refers to the flow condition within the river (specifically, the Froude number is less than 1). It relates to the depth and velocity of the water but doesn't directly describe the river's tendency to erode or deposit based on sediment availability.

Sediment Load vs. Transport Capacity

The core concept is the relationship between sediment load and transport capacity:

$Sediment \ Load < Sediment \ Transport \ Capacity$

When this inequality holds true, it signifies a sediment deficit. The river has the power (capacity) to carry more sediment than it is actually carrying (load). This excess power drives the process of bed erosion, leading to the river becoming degrading.

Therefore, a deficit of sediments in flowing water causes a river to become of the degrading type, as it actively erodes its channel bed to meet its transport capacity.