From the PFBC web site:
Life history: Carp spawn in late spring to early summer, over aquatic vegetation. They may choose a shallow, weedy bay. After rains have swelled their home river over its banks, they may move into flooded fields to deposit eggs on submerged plants. The splashing of their spawning commotion in shallow water can often be seen and heard. Several males may spawn with a female, which can release up to two million tiny eggs. The carp parents abandon the eggs. The eggs adhere to submerged vegetation and to the bottom. They hatch in four or five days. Carp grow to four or five inches their first year. They mature in three or four years, and they can live to be about 20 years old. The carp is an omnivore, eating a wide variety of aquatic plants, algae, insect larvae and other invertebrates, and even small fish. Its usual feeding method is to disturb the bottom material with its snout and pick up the food it dislodges, usually kicking up clouds of silt. Carp have a well-developed sense of taste and a sensitive mouth.
As it says, you often can hear loud splashes when they are spawning.