THE RELATIONSHIP BETWEEN ACTIVE CHANNEL AND CHANNEL BELT WIDTHS AND SINUOSITIES, AND HOW IT CHANGES DOWN A MEGAFAN

To examine whether a relationship exists between active channel width and current channel belt width we evaluated the channel belt system on the Rio Bermejo megafan, located in the Chaco Plain Basin. We defined the active channel as the portion of the river displaying either water or bars with minimal vegetation, and the current channel belt as the area where the active channel has recently migrated and characterized by younger vegetation and inactive bar features connected to the active channel. The Rio Bermejo can be divided into three zones based on active channel width: (1) from the apex to ~90 km below the apex, where it averages 2500 m width, varying between 500 and 4500 m, (2) between 90 and 160 km below the apex, where the active channel steadily decreases to ~ 500 m width, and (3) from 160 km to the confluence with the Rio Paraguay ~650 km below the apex, where the active channel steadily decreases in width from 500 m to ~200 m. Active channel sinuosity remains relatively constant, at ~1.1, from the apex until ~115 km from the apex. Below this point, mean sinuosity increases to 1.7, a change that appears to be related to the decrease in active channel size.

The Rio Bermejo’s current channel belt also displays a net decrease in width down-fan but shows higher variability. Up to 75 km from the apex the mean current channel belt width is ~4000 m. Between 75 and 120 km below the apex mean width decreases to ~ 3000 m. Below this point, the current channel belt mean width further decreases to ~2000 m, however significant variability is observed with a range between 1000-3000 m. Sinuosity of the channel belt is relatively constant and near the straight value of 1.0 and never exceeding 1.5. The relationship between channel belt width (CB) and active channel width (AC) on the Rio Bermejo was evaluated as a ratio (CB/AC) and evaluated against increasing distance down-fan. CB/AC increases with distance down-fan, although the variability of this relationship makes prediction difficult. Further research on other megafans of the Chaco Plain Basin, and in other basins, is necessary to determine if the trends we observed are similar on other megafans.