Tie Channels on Deltas: A Case Study from the Huanghe (Yellow River) Delta, China

On river floodplains, tie channels convey sediment-laden water between the main channel and adjacent floodplain waterbodies. Field-based studies have documented that the bidirectional nature of flow in tie channels evacuates sediment deposited within the channel, thereby preserving an approximately consistent geometry over time. Herein, these findings are tested for an alternative fluvial environment: a deltaic system, where the distributary channel of an abandoned lobe is now occupied by a subordinate tie channel. The study region is the Yellow River delta of China; specifically, a lobe abandoned ten years ago due to a natural avulsion. An active tie channel maintains a connection between the main river and the adjacent Bohai Sea; therefore, water flux arises due to both riverine and tidal inputs. Measurements of the tie channel location, collected using remote sensing data, indicate that it has migrated laterally several hundred meters since inception, while concomitantly width has reduced by a factor of ten. Several field data sets were collected to constrain the morphological evolution of this tie channel, and compare formative processes to those known for floodplain tie channels. These data include sediment cores, measurements of water stage and flow velocity within the tie channel, and detailed elevation surveys of both the tie and abandoned distributary channels. Preliminary analyses show that, under low to moderate riverine discharge, tides are the primary driver for changes in water stage and flow velocity within the tie channel; additionally, sedimentation arises at the main river and tie channel junction, nearly impeding movement of water between the two channels. This observation raises the question: What maintains the tie channel as an open flow conduit? It is likely that during river floods, enhanced water flux removes this sediment, thereby maintaining the tie channel as an open flow path. However, such floods have not occurred in the past two years, and there is notable encroachment of vegetation into the tie channel. Unlike oxbow tie channels, bi-directional flow may not be required to maintain geometry of deltaic tie channels; rather, it is the episodic movement of water during floods that maintains the channel over time, with narrowing due to vegetation growth during prolonged low flow periods.