A stream is at grade when

• A. Erosion is occurring but deposition is not
• B. Deposition is occurring but erosion is not
• C. Both erosion and deposition are occurring
• D. Neither eroding nor depositing material

Answer: (D)

A graded stream is in dynamic equilibrium where the channel has adjusted so its ability to move sediment matches the amount of sediment being supplied. In this state, erosion and deposition happen at the same time, balancing each other over long timescales so there is no net change in the valley profile. The river continuously erodes where the water is moving fast enough to carry material and deposits where velocity slows, forming bars, point bars, or terrace features, until the overall sediment budget and slope are just enough to transport the load steadily downstream. If there were no erosion or no deposition, the stream would not be able to adjust and would drift away from equilibrium; if one process dominated, the landscape would either aggrade or incise until balance is restored. Therefore, the best description is that both erosion and deposition are occurring.