Hydrogeomorphology 101 at Trestles

25 03 2010

Some may think that is a big word for surfers.   But, Trestles surfers probably understand this topic better than most people.  That’s because we surf everyday in a physics laboratory where this concept is at work.

Hydrogeomorphology is the study of how land forms are changed by the flow of water over, under, and through them.  San Mateo Creek, as one of the last pristine watersheds in Southern California, wends its way through the hills and coastal plains, carrying those same landforms with it to the ocean.  In this post, let’s look at San Mateo’s hydrogeomorphology at a macro scale.   In coming days, we will drill down to a more micro level.

At the macro scale, San Mateo Creek “braids” its way through Camp Pendleton.  In wet years, the alignment of the creek bed changes from one side of the flood plain to the other as it meets the resistance of downstream boulders, logs, and vegetation.  Some years it hugs the cliff near Cristianitos.  The next year it may flop over the south bank, eating away at the old tomato fields further into the base.  If the area is ever developed (like with a tollroad) and the creek is channelized, the beauty of the floodplain will be reduced to a thin ribbon instead of a rich habitat-covered canvas.  And, the natural processes will be adulterated.

San Mateo Creek Flowing to the Sea

Further downstream, where the creek meets the ocean, an estuary is formed.   While there is some variation in definitions, an estuary is where fresh water meets ocean water.  In that zone, the biological resources are rich.  Have you ever noticed how the flocks of seagulls and terns sit on the rocks  where the creek flows onto the beach?   They are looking for that free meal.  Estuaries generally have a free exchange of salt and fresh water, especially when the tides surge water back and forth.  That can happen when the creek has broken through the berm and flows freely.   That is what has been going on for the last two months.

On Tuesday, the creek stopped flowing to the ocean, at least on the surface.   The only creek water making it to the sea will flow underground through the sand berm.  So, before our very eyes the mouth of San Mateo Creek will transform from a tidal wetland/estuary into a natural lagoon.  The emergent marsh vegetation (cattails and bulrush) will return around the perimeter and eventually surround the lagoon, blocking it from view, until the next El Nino storms blow out the berm.

The Rebirth of the Lagoon -- No More Flow to the Sea

As you watch geomorphology in action in the coming months, keep an eye on the flora and fauna as well.  Have the steelhead safely made it upstream through the lagoon to their spawning grounds?  Have waters stilled enough to allow the turtles to poke their heads above the mirror-like water surface?  Oh, and will the beavers start building new lodges for their families?

In future posts, we’ll move on to how other hydrogeomorphologic forces work in our own backyard.  Rock on!

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