LEAKY RIVERS: IMPLICATIONS OF THE LOSS OF FLUVIAL DISCONNECTIVITY

Mountain rivers in the Colorado Front Range have at least three scales of naturally occurring disconnectivity. (1) Basin-scale disconnectivity at along-stream distances of 10²-10³ m driven by bedrock structure is expressed as downstream alternations between bedrock gorges with single channels and wider valley segments with anastomosing channels; this disconnectivity in valley geometry likely persists for 10³-10⁵ years. (2) Reach-scale disconnectivity at lengths of 10¹-10² m driven by channel-spanning logjams and overbank flow, which likely persists for 10¹-10² years. (3) Unit-scale disconnectivity at lengths of 10⁰-10¹ m driven by channel-spanning logjams and in-channel storage, which likely persists for 10⁰-10¹ years. Anastomosing channel reaches contain substantially more wood, channel-spanning logjams, fine sediment and organic matter than reaches with a single channel. Anastomosing channel planforms and associated organic storage occur only in old-growth (> 250 yr) forest stands that contain greater basal area of standing trees and larger diameter trees, suggesting a threshold effect between wood volume/log diameter and formation of channel-spanning logjams that force overbank flow and multiple channels. In the absence of old-growth forests and associated logjams, reach- and unit-scale disconnectivity decrease, causing a decrease in channel substrate heterogeneity, aquatic habitat diversity, and carbon storage and biochemical processing of organic matter. The existence of a threshold for channel characteristics driven by forest dynamics suggests the existence of alternate stable states; old-growth forests with abundant wood recruitment and storage in channels, numerous channel-spanning jams, multiple channels, and storage of organic matter, versus younger forests with limited wood recruitment and in-channel storage, fewer jams, single channels, and less organic matter storage.