A series of processes involving sediment delivery from channels, its dispersal onto the floodplain, and storage at their channel margins, often generates natural levees. When levees breach, they release water and sediment onto the floodplain, leading to crevasse splays or avulsions. Despite their importance in constructing floodplains and influencing channel mobility, levee growth is poorly understood. Presently, no model fully explains dynamic channel-levee evolution. A common assumption is that levee and channel bed aggradation rates are coupled. If they coevolve, levees should progressively accumulate along any aggrading channel belt, yet observations indicate otherwise. Using a one-dimensional numerical model, we investigate levee growth decoupled from channel bed aggradation across two flood scenarios wherein the flooded level: 1) exceeds the levee crest height (i.e., front loading); 2) is lower than the levee crest causing partial inundation of distal levee deposits (i.e., back loading). Initially, rapid levee aggradation confines the channel and increases bankfull depth, which mitigates flooding. As confinement slows levee growth, channel bed aggrades until bankfull depth is sufficiently reduced to trigger overflows. This releasing process increases flood likelihood and enhances overbank accumulation, promoting front loading and re-confining the channel. Our findings suggest aggradational channels may experience confined-release phases, characterized by episodic levee growth and fluctuations in bankfull depth. Rapid in-channel aggradation increases flood frequency and variability with more confined-release cycles. These results imply that river avulsions and associated floods might preferentially occur when the channel bed aggrades faster than adjacent levees, whereby the channel becomes shallower and destabilized.