Plastic pollution in aquatic ecosystems is a global problem. Rivers transport large quantities of litter from land to the oceans. Plastics of different sizes and properties are widely present at various locations in river systems. Concerning macroplastics, previous studies mainly focussed on floating plastics, however, substantial parts of the litter can be transported underneath the water surface. Currently, this suspended litter load remains understudied. Submerged litter is presently monitored with the use of nets. The use of nets has several disadvantages. In large rivers, nets can usually, not be deployed over the full depth and width of the river. Moreover, using nets is labour intensive and requires fixed structures to be deployed. To overcome this problem, this thesis aimed to develop and test a new method for monitoring submerged riverine litter. In accordance with the fish detection abilities of sonar, it seems to have a high potential for identifying suspended underwater objects.

In this study, the litter detection ability of a low-cost single beam echosounder (Deeper CHIRP+) were investigated. Three different experiments were executed, in specific, controlled tests in an artificial environment, semi-controlled tests in a natural environment and litter monitoring in a naturally flowing river. The controlled tests, to get an insight into the scanning technique and detection abilities of the echosounder, were performed in the Kerkpolderbad in Delft. During these tests, the influence of actual object size, object depth and flow velocity on the sonar signal was investigated. The semi-controlled tests were carried out in the Rio de San Pedro, in Andalusia, Southern Spain. During these tests, several plastic targets were used and repeatedly released in the river, passing the sensor. For this, objects of different material properties and sizes were used. Lastly, plastic was monitored in the Guadalquivir and Guadalete rivers in Andalusia. In the Guadalquivir river, the sensor was operated together with nets for validation purposes. In the Guadalete river, monitoring took place for 18 hours from a pedestrian bridge, at different locations over the cross-section of the river and under varying tidal conditions.

The performed tests showed a significant relationship between the dimensions of the reflection signal, derived from the sonar observations and the actual object size. However, object orientation and deformation play a role and lead to deviations in the signal dimension results. A second relation, regarding flow velocity and signal dimensions, was observed. The larger the flow velocity, the smaller the sonar signal. Additionally, signal intensities can, for four out of the eight objects tested, be related to material properties but differences in signal intensities are relatively small. Regarding the river monitoring activities, suspended litter items can be counted, river tide is influencing litter transport and litter is present over the full river depth.

The following main conclusions are drawn based on this research:
- Echo sounding can be used for detecting suspended riverine macroplastics. Litter items can be counted, and fish can be discarded from the sonar readings by their specific displayed shapes.
- Litter size can be estimated when looking at the sonar readings, however, several factors, such as flow velocity, object orientation and deformation have to be taken into account when estimating litter size.
- In the Guadalete river, significantly more suspended litter is transported when river water flows into the sea compared to river water flowing inland. The counted litter items were approximately uniformly distributed over the river depth.

In general, using echo sounding for suspended litter monitoring is potentially useful to gain a better understanding of the suspended litter transport, from which prevention and mitigation strategies could be optimised. For further research, it is recommended to use an echosounder for which the raw sonar data can be exported as a standard digital file. Moreover, the set of test objects should be extended, including more variation in object size. To separate signal size and signal intensity, objects of different size but same material properties and objects of the same size but different material properties should be used for testing. Finally, other types of sonar such as side scan or multibeam sonars may potentially lead to more accurate sonar readings regarding litter size and material estimations.