Several studies of sediment (e.g., dirt, pieces of rock, etc.) in streams have shown an inverse correlation between sediment grain...
GMAT Information and Ideas : (Ideas) Questions
Several studies of sediment (e.g., dirt, pieces of rock, etc.) in streams have shown an inverse correlation between sediment grain size and downstream distance from the primary sediment source, suggesting that stream length has a sorting effect on sediment. In a study of sediment sampled at more than a dozen sites in Alpine streams, however, geologists Camille Litty and Fritz Schlunegger found that cross-site variations in grain size were not associated with differences in downstream distance, though they did not conclude that downstream distance is irrelevant to grain size. Rather, they concluded that sediment influx in these streams may have been sufficiently spatially diffuse to prevent the typical sorting effect from being observed.
Which finding about the streams in the study, if true, would most directly support Litty and Schlunegger's conclusion?
The streams regularly experience portions of their banks collapsing into the water at multiple points upstream of the sampling sites.
The streams contain several types of sediment that are not typically found in streams where the sorting effect has been demonstrated.
The streams mostly originate from the same source, but their lengths vary considerably due to the different courses they take.
The streams are fed by multiple tributaries that carry significant volumes of sediment and that enter the streams downstream of the sampling sites.
Step 1: Decode and Map the Passage
Passage Analysis Table
| Text from Passage | Analysis |
|---|---|
| "Several studies of sediment (e.g., dirt, pieces of rock, etc.) in streams have shown an inverse correlation between sediment grain size and downstream distance from the primary sediment source," |
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| "suggesting that stream length has a sorting effect on sediment." |
|
| "In a study of sediment sampled at more than a dozen sites in Alpine streams, however, geologists Camille Litty and Fritz Schlunegger found that cross-site variations in grain size were not associated with differences in downstream distance," |
|
| "though they did not conclude that downstream distance is irrelevant to grain size." |
|
| "Rather, they concluded that sediment influx in these streams may have been sufficiently spatially diffuse to prevent the typical sorting effect from being observed." |
|
Passage Architecture & Core Elements
Main Point: While most studies show sediment gets finer downstream, Litty and Schlunegger found this pattern absent in Alpine streams due to sediment entering at many different points.
Argument Flow: The passage establishes normal scientific understanding about sediment sorting, then presents conflicting evidence from Alpine streams, and concludes with an alternative explanation.
Step 2: Interpret the Question Precisely
What's being asked? Which finding would most directly support Litty and Schlunegger's specific conclusion about spatially diffuse sediment influx.
What type of answer do we need? Evidence that would strengthen their explanation that sediment is entering the streams at multiple points rather than from one primary source.
Any limiting keywords? "Most directly support" means we need the choice that provides the strongest, most relevant evidence for their conclusion.
Step 3: Prethink the Answer
- Litty and Schlunegger concluded that "spatially diffuse" sediment influx prevented normal sorting
- This means sediment was entering the streams at many different locations rather than flowing primarily from one upstream source
- For this to be true, we'd need evidence of:
- Multiple sediment sources along the stream
- Sediment entering at various points, not just the headwaters
- Something disrupting the normal "primary source to downstream flow" pattern
- So the right answer should show that sediment was indeed entering these Alpine streams from multiple locations throughout their length
The streams regularly experience portions of their banks collapsing into the water at multiple points upstream of the sampling sites.
- Bank collapses at multiple upstream points would create exactly the "spatially diffuse" sediment influx L&S described
- Instead of sediment coming primarily from the headwaters, it would enter the stream at numerous locations
- This directly explains why normal downstream sorting didn't occur - new sediment kept entering at different points
The streams contain several types of sediment that are not typically found in streams where the sorting effect has been demonstrated.
- Different sediment types doesn't address where the sediment enters the stream
- Having unusual sediment composition doesn't support the spatial diffusion explanation
- Trap: Students might think "different" sediment explains unexpected results, but the issue isn't sediment type - it's sediment distribution
The streams mostly originate from the same source, but their lengths vary considerably due to the different courses they take.
- Streams having the same source but different lengths actually contradicts spatial diffusion
- This describes a single source scenario, not multiple input points
- Different stream lengths wouldn't prevent normal sorting if sediment came from one primary source
The streams are fed by multiple tributaries that carry significant volumes of sediment and that enter the streams downstream of the sampling sites.
- Tributaries downstream of sampling sites wouldn't affect the sediment samples already collected
- The sediment in the samples would still reflect upstream sources and sorting
- Trap: Students might think multiple tributaries equals multiple sources, but these enter after the sampling occurred