Streams and rivers transport sediment across the landscape by eroding in one location and depositing in another. A common model for understanding sediment transport in streams in Lane’s Balance:
In mathematical terms, this is a proportional relationship between sediment load in the stream (Q s) and stream flow (Qw), where D is sediment size and S is the slope of the stream:
Qs D ∝ Qw S
In other words, changes to one variable of the balance such as increased flows from snowmelt or decreased sediment load due to bank armor provokes changes to the other variables to maintain the balance. Increased flows have the capacity to carry a larger volume of sediment (increasing Qs) as well as the competence to erode larger sediment particles (increasing D).
Let’s say we armor a section of the bank with rip-rap. This decreases the sediment load (Qs) since rip-rap by design prevents erosion. Rip-rap tends to increase the velocity of water by reducing friction with the banks, increasing the capacity and competence of the stream. When this fast and sediment-deficient stream encounters an unprotected bank, the resulting erosion will be worse than if the stream had no rip-rap. Artificially straightening and channelizing the stream has similar consequences, but because of an increase to the slope.
Dams have complex effects on sediment transport. On one hand, dams capture upstream sediment (reducing Qs) and causing increased erosion downstream to replace lost sediment. On the other hand, dams reduce peak flows (Qw) that may have previously eroded sediment downstream. The net effect depends on the situation. On the Colorado River in the Grand Canyon below Glen Canyon Dam, these changes combined with the very steep slope tilt the balance towards degradation where the very fast sediment-starved water erodes everything it comes across without leaving any sediment behind. However in the Colorado River along the Arizona-California border, dams and the mild slope tilts the balance towards aggradation and excessive sediment buildup due to insufficient flows to cause erosion.
Streams are in a state of dynamic equilibrium, where they respond quickly to changes to reach a new equilibrium state. Any human activities that alter the variables in Lane’s Balance will cause downstream impacts, such as increasing damaging erosion events. Furthermore, the integrity of native ecosystems relies on the natural dynamic character of flows, sediments, and temperatures. Alterations to these variables will inevitably cause damage to the native ecosystem and invite replacement by invasive species.