If the trek is taking place in Catalonia then a 'riera' is a river or streambed that only fills with water after it has rained further inland or the snow on mountains is melting, even if the mountains are miles away.

In a seaside town in Catalonia, called Arenys de Mar, there's a 'riera' where people used to often park their cars. After heavy rain it was common to hear on the news that a number of cars had been swept away by the water in the 'riera'.

gul•ly (ˈgʌl i)

n., pl. -lies, n.
1. a small valley or ravine orig. worn away by running water and serving as a drainageway after prolonged heavy rains; gulch.
2. a ditch or gutter.
v.t.
3. to make gullies in.
4. to form (channels) by the action of water.

http://www.thefreedictionary.com/gully

http://geographyfieldwork.com/VendrellFloodPrevention5.htm




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Note added at 6 hrs (2016-06-21 15:08:06 GMT)
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https://www.google.co.uk/search?q=riera de la bisbal el vend...

http://www.meteored.com./ram/UserFiles/Image/Enero09/maresme...



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Note added at 6 hrs (2016-06-21 15:11:00 GMT)
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The Riera (the dry uncovered river bed) is the commercial centre and the hub of all local activities. The market, an Art Nouveau building, remains open every morning as well as Friday afternoons. Inside, it is bustling with people and constant activity. On Saturday mornings, the commercial hustle takes itself to the Riera in the form of a weekly street market. Two village festivals are held in honour of Saint Zeno (9 July) and Saint Roch (16 August), and a big fair is celebrated on Saint John's Day (24 June).

https://en.wikipedia.org/wiki/Arenys_de_Mar


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Note added at 7 hrs (2016-06-21 16:23:47 GMT)
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Also, 'gulch'

a small, narrow valley with steep sides
Source: Merriam-Webster's Learner's Dictionary
Examples: gulch in a sentence

: a deep or precipitous cleft : ravine; especially : one occupied by a torrent
See gulch defined for English-language learners

<the gulch floods in the spring with the runoff from the mountains, so wait until later in the summer to hike it>

http://www.merriam-webster.com/dictionary/gulch

A gulch is a deep V-shaped valley formed by erosion. It may contain a small stream or dry creek bed and is usually larger in size than a gully. Occasionally, sudden intense rainfall may produce flash floods in the area of the gulch.

http://forums.mtbr.com/california-norcal/gulches-gorges-cany...