How does groundwater move through rock and sediment?

Not through perfectly solid material. Groundwater can move through openings: spaces between sediment grains, or openings such as fractures in rock. The openings must connect for water to pass through them. Permeability describes how readily water can move through connected spaces. Well-sorted sand and gravel can have connected pores; compact rock can also transmit water if its fractures are sufficiently large and connected. [S1]

Here is a conceptual cross-section. It is not to scale and does not represent a particular location:

             higher head                         lower head
GROUND ───────────────────────────────────────────────────
       water table  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       unconfined aquifer: connected sand or gravel
                  → → → → → → → → →
       less-permeable layer (illustrative)
       ───────────────────────────────────────────────────
       confined aquifer: connected rock fractures
                  → → → → → →

The layers and arrows are illustrative placeholders. They do not mean every underground setting has these layers or flow paths.

For a basic flow diagram, point arrows from higher hydraulic head toward lower hydraulic head. Hydraulic head describes conditions that drive flow. In an unconfined aquifer, it can be understood in relation to water-table elevation; in a confined aquifer, pressure also matters. [S2]

“Downhill” can be misleading if it means following the land surface. The general direction described here is based on differences in head, not a rule that groundwater always follows the steepest surface slope. For a teaching diagram, label the ends “higher head” and “lower head,” then point the arrows between them. The USGS report discusses regional principles in the Upper Klamath Basin; that example does not establish conditions at another site. [S2]

Use this as a reusable teaching sketch. Choose layer names only as placeholders; the actual arrangement and connectivity depend on the setting.

CONCEPTUAL CROSS-SECTION — not to scale; not site-specific

higher head                                      lower head
────ground surface──────────────────────────────────────
    water table  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    connected sediment pores       → → → → →
────────────── less-permeable layer ───────────────────
    connected rock fractures      → → →

Glossary

1. Pore: An opening between sediment grains.
2. Fracture: An opening in rock; it can transmit water when sufficiently large and connected to other openings. [S1]
3. Permeability: How readily water can pass through connected openings. [S1]
4. Hydraulic head: A measure used to describe conditions that drive groundwater flow; flow is generally toward lower head. [S2]
5. Water table: In an unconfined aquifer, its elevation helps describe hydraulic head. In a confined aquifer, pressure also contributes. [S2]

The sketch conveys general principles, not a measured aquifer, exact flow rate, or local underground map.

Check that the picture separates two ideas: pathways and driving direction. Show connected pores or fractures as possible pathways; point flow arrows from higher head toward lower head. Avoid drawing arrows through a layer you have labeled as having no connected water pathways. If you do not know the real subsurface conditions, call the drawing conceptual rather than presenting it as a map.

A useful caption is: “Groundwater can move through connected pores or rock openings. Its general flow direction is from higher hydraulic head toward lower head. This sketch is illustrative and does not describe a specific site.” [S1] [S2]

These general concepts do not establish local geology, a site’s actual groundwater direction, or how quickly water moves there. Those questions require site-specific evidence beyond this diagram.