A combine harvester comprising: - a header for harvesting a crop of a field; - a threshing system for separating a crop residue from the harvested crop; and - a monitoring system comprising: - a sensor (10.1) configured to provide a measurement wave to a flow of the crop residue on the harvester and to receive a response wave from the flow of crop residue,; - a processor having an input terminal for receiving a response signal of the sensor (10.1) representative of the response wave; the processor being configured to determine a crop parameter associated with the density of the flow of crop residue based on the response signal of the sensor (10.1); - the processor further comprising an output terminal for outputting a density signal representing the crop parameter, characterized in that: the sensor (10.1) is an ultrasonic sensor (10.1) including a transmitter configured to transmit the measurement wave to the flow of crop residue and a receiver configured to receive the response wave from the flow of crop residue, the transmitter being configured to output an ultrasonic pulse as the measurement wave, the receiver being configured to receive, as the response wave, a response of the ultrasonic pulse after impacting the flow of crop residue; and the processor is configured to determine the crop parameter associated with the density of the flow of the crop residue using a method including at least the steps of: - identifying individual peaks in the response signal; - determining a metric directly related to a distance between consecutive ones of the identified individual peaks; and - using the determined metric for determining the crop parameter associated with the density of the flow of the crop residue. - a header for harvesting a crop of a field; - a threshing system for separating a crop residue from the harvested crop; and - a monitoring system comprising: - a sensor (10.1) configured to provide a measurement wave to a flow of the crop residue on the harvester and to receive a response wave from the flow of crop residue,; - a processor having an input terminal for receiving a response signal of the sensor (10.1) representative of the response wave; the processor being configured to determine a crop parameter associated with the density of the flow of crop residue based on the response signal of the sensor (10.1); - the processor further comprising an output terminal for outputting a density signal representing the crop parameter, characterized in that: the sensor (10.1) is an ultrasonic sensor (10.1) including a transmitter configured to transmit the measurement wave to the flow of crop residue and a receiver configured to receive the response wave from the flow of crop residue, the transmitter being configured to output an ultrasonic pulse as the measurement wave, the receiver being configured to receive, as the response wave, a response of the ultrasonic pulse after impacting the flow of crop residue; and the processor is configured to determine the crop parameter associated with the density of the flow of the crop residue using a method including at least the steps of: - a sensor (10.1) configured to provide a measurement wave to a flow of the crop residue on the harvester and to receive a response wave from the flow of crop residue,; - a processor having an input terminal for receiving a response signal of the sensor (10.1) representative of the response wave; the processor being configured to determine a crop parameter associated with the density of the flow of crop residue based on the response signal of the sensor (10.1); - the processor further comprising an output terminal for outputting a density signal representing the crop parameter, the sensor (10.1) is an ultrasonic sensor (10.1) including a transmitter configured to transmit the measurement wave to the flow of crop residue and a receiver configured to receive the response wave from the flow of crop residue, the transmitter being configured to output an ultrasonic pulse as the measurement wave, the receiver being configured to receive, as the response wave, a response of the ultrasonic pulse after impacting the flow of crop residue; and the processor is configured to determine the crop parameter associated with the density of the flow of the crop residue using a method including at least the steps of: - identifying individual peaks in the response signal; - determining a metric directly related to a distance between consecutive ones of the identified individual peaks; and - using the determined metric for determining the crop parameter associated with the density of the flow of the crop residue.