Form v. Process-Based Restoration
Illustration of a form-based restoration design whereby:
(a) Infrastructure constraints are removed using heavy equipment. In this example, infrastructure such as levees and an upstream culvert blocking fish passage are removed.
(b) Instream work is extensive and heavily engineered, requires heavy equipment, and a large carbon and disturbance footprint. In this example, an inset floodplain is created, which keeps the water table depressed and limits the amount of process space that can be reconnected.
(c) Extrinsic materials such as rock and large wood are brought in to help create a stable channel form that is in equilibrium with respect to sediment transport so that erosion and deposition within the reach are minimal. The inset floodplain area is designed with a single-thread bankfull channel typically flooding once every 1–2 years. Vegetation growth is suppressed because the groundwater level is rarely at or near the surface.
(d) The channel remains stable over time and opportunities for habitat-forming processes are limited.
Illustration of a process-based restoration design whereby:
(a) Infrastructure removal, some using heavy equipment, is an explicit focus to create as large an area as possible for natural habitat forming fluvial processes to occur. In this example, levees and an upstream culvert blocking fish passage are removed, cattle are removed temporarily, and beaver trapping ceases.
(b) Instream work may be extensive but relies on stream energy and natural materials and has a small carbon and disturbance footprint. In this case beaver dam analogues raise water tables to the surface of the floodplain and to encourage side channel formation and sediment deposition.
(c) Fluvial energy, sediment, and vegetation develop a complex multichannel system. A productive and biologically diverse system is created; beaver colonize the area, adding further hydrogeomorphic complexity; predators return.
(d) Over time the system remains dynamic, with habitat elements forming and disappearing, and reappearing elsewhere.