Geomorphic effects of wood quantity and characteristics in three Italian gravel-bed rivers

- In-channel wood shed light on the degree of human pressure at basin scale.
- Basins with less human pressures ensure higher wood recruitment to river systems.
- Resprouting capacity of logs is higher in less impacted systems.

In-channel wood is a fundamental component of the riverine system. Its nature, abundance, and distribution as well as the role of wood in trapping sediment have been reported by many authors. However, a lack of knowledge still exists on how the geomorphic effects, quantity, and characteristics of in-channel wood may be altered by different human pressures. For this reason, in-channel wood was surveyed in the Brenta, Piave, and Tagliamento gravel-bed rivers (northeastern Italy), which are altered by different degrees of human pressures. Both single pieces of wood (> 0.1 m diameter, and/or > 1 m long) and accumulations of large wood were measured on cross sectional transects within the active channels. Overall, 3430 (8.4, 13.9 and 10.7 elements/ha in the Brenta, Piave, and Tagliamento rivers, respectively) of isolated pieces and 591 (9.8, 15.0, and 11.0 wood accumulations/ha in the Brenta, Piave, and Tagliamento rivers, respectively) accumulations were surveyed in the study sites. In the Brenta and Piave rivers, which feature the greater human pressures, logs appear in a worse state of conservation. In the less disturbed Tagliamento River, the logs appear to be smaller and in a better state of conservation with higher capacity for resprouting. In addition, higher geomorphic interactions were found between wood and sediments in the Tagliamento River. Because of its ability to create geomorphic effects, in-channel wood represents an important source of complexity that can increase habitat diversity in river systems. A better knowledge of the role of human disturbances on the characteristics and abundance of large wood in river systems could help in developing better river management and the practical application of river ecology.