CALIBRATION OF RIVER LOAD DATA BY DELTA GROWTH DATA IN SEMI-ENCLOSED LAKES

Long term delta growth data combined with lake sedimentation rates provide relatively precise denudation rates for catchments. These data suggest that river load measurements may display a systematic underrating. In a case study of the Tiroler Ache and the Alpine Rhine (Alps, Europe), river load measurements underrate delta growth data by 25 to 30 %.

The underrating is probably due to the missing registration of the sand fraction in river load measurements. Depending on variable runoff and transport capacity of the river, the sand fraction can be part of the suspended load during flooding events and could be measured if sophisticated sampling techniques are applied. During normal discharge the sand fraction is part of the bedload and will pass standard samplers unregistered. Sediment volumes discharged during extreme runoff events are generally underrated.Unquantified excarvation of gravel upstream also potentially reduces the measured bedload.

On the other hand, delta growth data need to be carefully examined. Repeated depth measurements of the Alpine Rhine fan in eastern Lake Constance are performed since 130 years with changing methods. These reveal volume estimates difficult to transform into mass, since density or porosity data of the unconsolidated sediment are lacking. Standard values being used here are underestimating the density of the proximal delta facies. In the case of Lake Constance, sedimentation rates at the delta foot and the distal lake have to be extrapolated on the base of sedimentation rates in cores. If the distal parts of the lakes include turbidite sedimentation, a relatively high density of core samples is required to cover variable sediment thickness due to subaquatic morphology and the magnitude of extreme depositional events. The principle advantage of delta growth measurements are multiple and more precise methods, if applied, and the increasing robustness of measurements integrating over tens of years.