Why do rivers deposit sediment in some places and erode in others?

Think of erosion and deposition as two parts of a moving exchange. Flow can pick up and carry sediment; erosion changes the bed or banks, while deposition happens when the flow can no longer keep some of its load moving. Flow, sediment movement, and channel-bed shape affect one another, so no single feature decides the outcome on its own. [S1]

A useful first comparison is:

Steeper, narrower reach                 Flatter, wider reach
Flow conditions to assess                Flow conditions to assess
        ↓                                         ↓
Look for signs of erosion               Look for possible deposition

That is a hypothesis to check, not a universal rule. The actual result also depends on how much water and sediment arrive and on channel geometry. [S1] [S2] In a drawing, label what you can observe separately from what you infer: for example, “channel widens” is an observation; “deposition is likely here” is an interpretation.

Use several clues together. For possible deposition, look for a change from steep to gentle slope, a channel that widens, or flow that is obstructed. These settings can reduce flow velocity or its ability to carry sediment. For possible erosion, ask whether the flow appears able to dislodge material from the bed or banks; particles need sufficient force to be moved. [S2]

Particle size matters too: larger particles settle from suspension faster than smaller ones, so a reach may deposit coarse material while finer sediment continues downstream. [S2] Don’t assume that every visible bend, widening, or obstacle proves deposition, or that all sizes settle in the same place. Compare the clue with the direction of flow and the material shown, if the illustration includes it.

Suppose the illustration shows water flowing from a narrow, steep channel into a broader, gentler reach. You could annotate it like this:

UPSTREAM                                             DOWNSTREAM
[narrow + steep]  →  [wider + gentler]
Possible erosion      Possible deposition
Check: flow can       Check: flow may slow or
move material?        lose carrying capacity?

Add a short note beneath it: “The change in slope and channel width makes deposition a possibility here. The diagram alone does not establish the water’s speed, sediment supply, or whether every particle settles.” The wording keeps the observation, inference, and uncertainty distinct. A field guide describing incised channels identifies these features as likely depositional settings, but treats them as tendencies rather than guarantees for every river reach. [S2]

If the drawing includes a barrier, debris, vegetation, or a sudden change in channel form, mark it as another possible influence on flow rather than as proof. Channel form and flow interact, and river behavior also depends on water and sediment supplies. [S1]

For each section, make a small evidence-and-inference note instead of assigning a label from one clue alone:

Section: [name or location]
Observed: [slope, width, obstacle, or material shown]
Flow clue: [what might change about flow?]
Sediment clue: [what material is shown or may be carried?]
Interpretation: [erosion / deposition / uncertain]
Reason: [connect the interpretation to the observations]

Then compare the sections. A change in slope, width, or obstacles may affect flow and favor deposition, but it does not determine the outcome by itself; channel shape, flow, and sediment supply work together. [S1] [S2] If the illustration lacks information about flow speed or sediment, write “uncertain” rather than filling in missing details. That makes the comparison useful without claiming more than the diagram shows.