Final answer:
To test if small sand grains lead to narrow, deep rivers, Armando can use a model to vary sand grain size and observe sediment movement and river morphology changes. Ripple marks help indicate water flow and river energy, while groove casts provide insights into sediment transport direction.
Step-by-step explanation:
Armando could use a model to test the idea that very small sand grains lead to narrow, deep rivers by simulating river flow and sediment movement in different scenarios. By varying the size of the sand grains in his model, Armando could observe the effect of grain size on the river's depth and width. In the physical model, he could manipulate the flow rate of water and the size of sand particles used as sediment, to witness how smaller grains might be more easily carried by the water, potentially leading to greater erosion and possibly resulting in narrower and deeper river channels. Additionally, the ripple marks formed in these models could indicate the flow direction and the relative energy of the water, which in combination with the sand grain size, affects the river morphology.
Armando could also compare the modern and ancient examples of mudcracks from the provided figures, understanding how sediment fills in and preserving these features. This comparison can shed light on past river dynamics, including the size of sediments involved. Furthermore, observing groove casts and erosional features, as shown in the figures provided, can help Armando extrapolate how sediment size might influence river formation and change over time.